![two lobes, the lobi inferiores (Plate 37, fig. 46E, l.in.), which are
continued posteriorly into the crura cerebri.
The pituitary body[516]
(Plate 37, figs. 44, 45, 46E, pt.) is small, not
divided into lobes, and provided with a very minute lumen.
In front of the infundibulum is the optic chiasma (Plate 37, fig. 46D,
op.ch.), which is developed very early. It is, as stated by Müller, a
true chiasma.
The mid-brain (Plate 37, figs. 44 and 45, m.b.) is large, and consists
in both stages of (1) a thickened floor forming the crura cerebri, the
central canal of which constitutes the iter a tertio ad quartum
ventriculum; and (2) the optic lobes (Plate 37, figs. 46E, F, G, op.l.)
above, each of which is provided with a cavity continuous with the
median iter. The optic lobes are separated dorsally and in front by a
well-marked median longitudinal groove. Posteriorly they largely
overlap the cerebellum. In the anterior part of the optic lobes, at the
point where the iter joins the third ventricle, there may be seen
slight projections of the floor into the lumen of the optic lobes (Plate
37, fig. 46E). These masses probably become in the adult the more
conspicuous prominences of the floor of the ventricles of the optic
lobes, which we regard as homologous with the tori semicirculares of
the brain of the Teleostei.
The hind-brain is formed of the usual divisions, viz.: cerebellum and
medulla oblongata (Plate 37, figs. 44 and 45, cb., md.). The former
constitutes a bilobed projection of the roof of the hind-brain. Only a
small portion of it is during these stages left uncovered by the optic
lobes, but the major part extends forwards for a considerable
distance under the optic lobes, as shewn in the transverse sections
(Plate 37, figs. 46F and G, cb.); and its two lobes, each with a
prolongation of its cavity, are continued forwards beyond the
opening of the iter into the fourth ventricle.
It is probable that the anterior horns of the cerebellum are
equivalent to the prolongations of the cerebellum into the central](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-21-320.jpg)
![The large size of the thalamencephalon is also characteristic of the
Ganoid brain in comparison with the brain of the Dipnoi; but is not
however so very much more marked in the Ganoids than it is in
some Elasmobranchii.
On the whole, we may consider the retention of a large
thalamencephalon as a primitive character.
The second feature which we have given as characteristic of the
Ganoid brain is essentially that which has been insisted upon by
Wilder, though somewhat differently expressed by him.
The simplest condition of the cerebrum is that found in the larva of
Lepidosteus, where there is an anterior pair of lobes, and an
undivided posterior portion with a simple prolongation of the third
ventricle, and a very thin roof. The dorsal edges of the posterior
portion, adjoining the thin roof, usually become somewhat everted
(cf. Wilder), and in Lepidosteus these edges have in the adult a very
great development, and form (vide Plate 38, fig. 47A-C, ce´.) two
prominent lobes, which we have spoken of as the posterior cerebral
lobes.
These characters of the cerebrum are perhaps even more distinctive
than those of the thalamencephalon.
In Teleostei the cerebrum appears to be completely divided into two
hemispheres, which are, however, all but solid, the lateral ventricles
being only prolonged into their bases. In Dipnoi again there is either
(Protopterus, Wiedersheim[517]) a completely separated pair of oval
hemispheres, not unlike those of the lower Amphibia, or the oval
hemispheres are not completely separated from each other
(Ceratodus, Huxley[518], Lepidosiren, Hyrtl[519]); in either case the
hemispheres are traversed for the whole length by lateral ventricles
which are either completely or nearly completely separated from
each other.
In Elasmobranchii the cerebrum is an unpaired though bilobed body,
but traversed by two completely separated lateral ventricles, and](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-24-320.jpg)
![indicated, in the higher Amphibia. The presence of the peculiar
vesicle attached to the roof of the thalamencephalon has its parallel
in the brain of Protopterus, and as pointing in the same direction a
general similarity in the appearance of the brain of Polypterus to that
of the Dipnoi may be mentioned.
There appears to us to be in no points a close resemblance between
the brain of Ganoids and that of Elasmobranchii.
[509] The homologies of the olfactory lobes throughout the group of
Fishes require further investigation.
[510] “Ueb. d. Gehirn des Störs,” Müller's Archiv, 1843, and
Lehrbuch d. vergl. Anat. d. Wirbelthiere. Cattie, Archives de Biologie,
Vol. III. 1882, has recently described in Acipenser sturio a vesicle on
the roof of the thalamencephalon, whose cavity is continuous with
the third ventricle. This vesicle is clearly homologous with that in
Lepidosteus. (June 28, 1882.)
[511] R. Wiedersheim, Morphol. Studien, 1880, p. 71.
[512] “On Ceratodus Forsteri,” c., Proc. Zool. Soc. 1876.
[513] In Wilder's figure the walls of the cerebellum are represented
as much too thin.
[514] Vide F. M. Balfour, Comparative Embryology, Vol. II. figs. 248
and 250.
[515] Vide F. M. Balfour, Comparative Embryology, Vol. II. pp. 355-6
[the original edition], where it is suggested that this commissure is
the homologue of the grey commissure of higher types.
[516] We have not been able to work out the early development of
the pituitary body as satisfactorily as we could have wished. In Plate
37, fig. 40, there is shewn an invagination of the oral epithelium to
form it; in Plate 37, figs. 41 and 42, it is represented in transverse
section in two consecutive sections. Anteriorly it is still connected
with the oral epithelium (fig. 41), while posteriorly it is free. It is
possible that an earlier stage of it is shewn in Plate 36, fig. 35. Were
it not for the evidence in other types of its being derived from the
epiblast we should be inclined to regard it as hypoblastic in origin.
[517] Morphol. Studien, III. Jena, 1880.](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-26-320.jpg)
![[518] “On Ceratodus Forsteri,” Proc. Zool. Soc. 1876.
[519] Lepidosiren paradoxa. Prag. 1845.
Sense Organs.
Olfactory organ.
Development.—The nasal sacks first arise during the late embryonic
period in the form of a pair of thickened patches of the nervous layer
of the epiblast on the dorsal surface of the front end of the head
(Plate 37, fig. 39, ol.). The patches very soon become partially
invaginated; and a small cavity is developed between them and the
epidermic layer of the epiblast (Plate 37, figs. 42 and 43, ol.).
Subsequently, the roof of this space, formed by the epidermic layer
of the epiblast, is either broken through or absorbed; and thus open
pits, lined entirely by the nervous layer of the epidermis, are formed.
We are not acquainted with any description of an exactly similar
mode of origin of the olfactory pits, though the process is almost
identical with that of the other sense organs.
We have not worked out in detail the mode of formation of the
double openings of the olfactory pits, but there can be but little
doubt that it is caused by the division of the single opening into two.
The olfactory nerve is formed very early (Plate 37, fig. 39, I), and, as
Marshall has found in Aves and Elasmobranchii, it arises at a stage
prior to the first differentiation of an olfactory bulb as a special lobe
of the brain.
The Eye.
Anatomy.—We have not made a careful histological examination of
the eye of Lepidosteus, which in our specimens was not sufficiently](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-27-320.jpg)
![of the vitreous humour is shewn in Plate 38, fig. 48 (vs.); its relation
to the optic nerve in Plate 37, fig. 46, C and D (vs.).
The above sheath has, so far as we know, its nearest analogue in
the eye of Alytes, where, however, it is only found in the larva.
The reader who will take the trouble to refer to the account of the
imperfectly-developed processus falciformis of the Elasmobranch eye
in the treatise On Comparative Embryology, by one of us[520]
, will not
fail to recognize that the folds of the retina at the sides of the
choroid slit, and the mesoblastic process passing through this slit,
are strikingly similar in Lepidosteus and Elasmobranchii; and that, if
we are justified in holding them to be an imperfectly-developed
processus falciformis in the one case, we are equally so in the other.
Johannes Müller mentions the absence of a processus falciformis as
one of the features distinguishing Ganoids and Teleostei. So far as
the systematic separation of the two groups is concerned, he is
probably perfectly justified in this course; but it is interesting to
notice that both in Ganoids and Elasmobranchii we have traces of a
structure which undergoes a very special development in the
Teleostei, and that the processus falciformis of Teleostei is therefore
to be regarded, not as an organ peculiar to them, but as the peculiar
modification within the group of a primitive Vertebrate organ.
[520] Vol. II. p. 414 [the original edition].
Suctorial Disc.
One of the most remarkable organs of the larval Lepidosteus is the
suctorial disc, placed at the front end of the head, to which we have
made numerous allusions in the first section of this memoir.
The external features of the disc have been fully dealt with by
Agassiz, and he also explained its function by observations on the](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-30-320.jpg)
![are, however, except on the tail, much less prominent than the
dorsal ridges.
The changes which next ensue are practically identical with those
which take place in Teleostei. Around the cuticular sheath of the
notochord there is formed an elastic membrane—the membrana
elastica externa. At the same time the basal parts of the dorsal, or
as we may perhaps more conveniently call them, the neural ridges of
the notochord become enlarged at each intermuscular septum, and
the tissue of these enlargements soon becomes converted into
cartilage, thus forming a series of independent paired neural
processes riding on the membrana elastica externa surrounding the
notochord, and extending about two-thirds of the way up the sides
of the medullary cord. They are shewn in transverse section in Plate
41, fig. 67 (n.a.), and in a side view in fig. 68 (n.a.).
Simultaneously with the neural arches, the hæmal arches also
become established, and arise by the formation of similar
enlargements of the ventral or hæmal ridges. In the trunk they are
very small, but in the region of the tail their condition is very
different. At the front end of the anal fin the paired hæmal arches
suddenly enlarge and extend ventralwards (Plate 41, fig. 67, h.a.).
Each succeeding pair of arches becomes larger than the one in front,
and the two elements of each arch first nearly meet below the
caudal vein (Plate 41, fig. 67) and finally actually do so, forming in
this way a completely closed hæmal canal. At the point where they
first meet the permanent caudal fin commences, and here (Plate 41,
fig. 68) we find that not only do the hæmal arches meet and
coalesce below the caudal vein, but they are actually produced into
long spines supporting the fin-rays of the caudal fin, which thus
differs from the other fins in being supported by parts of the true
vertebral column and not by independently formed elements of the
skeleton.
Each of the large caudal hæmal arches, including the spine, forms a
continuous [TN18] whole, and arises at an earlier period of larval life](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-36-320.jpg)
![than any other part of the vertebral column. We noticed the first
indications of the neural arches in the larva of about a week old,
while they are converted into fully formed cartilage in the larva of
three weeks.
The neural and hæmal arches, resting on the membrana elastica
externa, do not at this early stage in the least constrict the
notochord. They grow gradually more definite, till the larva is five or
six weeks old and about 26 millims. in length, but otherwise for a
long time undergo no important changes. During the same period,
however, the true sheath of the notochord greatly increases in
thickness, and the membrana elastica externa becomes more
definite. So far it would be impossible to distinguish the development
of the vertebral column of Lepidosteus from that of a Teleostean
Fish.
Of the stages immediately following we have unfortunately had no
examples, but we have been fortunate enough to obtain some
young specimens of Lepidosteus[521], which have enabled us to work
out with tolerable completeness the remainder of the developmental
history of the vertebral column. In the next oldest larva, of about 5.5
centims., the changes which have taken place are already sufficient
to differentiate the vertebral column of Lepidosteus from that of a
Teleostean, and to shew how certain of the characteristic features of
the adult take their origin.
In the notochord the most important and striking change consists in
the appearance of a series of very well marked vertebral
constrictions opposite the insertions of the neural and hæmal
arches. The first constrictions of the notochord are thus, as in other
Fishes, vertebral; and although, owing to the growth of the
intervertebral cartilage, the vertebral constrictions are subsequently
replaced by intervertebral constrictions, yet at the same time the
primitive occurrence of vertebral constrictions demonstrates that the
vertebral column of Lepidosteus is a modification of a type of
vertebral column with biconcave vertebræ.](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-37-320.jpg)
![development is wholly independent of the true neural arches, and
which constitutes the median spinous element of the adult. In
tracing these backwards it becomes obvious that they are
homologous with the interspinous elements supporting the dorsal
fin, in that they are replaced by these interspinous elements in the
region of the dorsal fin, and that the interspinous bones occupy the
same position as the median spinous processes. This homology was
first pointed out by Götte in the case of the Teleostei.
Immediately beneath this rod is placed the longitudinal ligament
(Plate 41, fig. 70, l.l.), but there is as yet no trace of a junction
between the neural arches of the two sides in the space between
the longitudinal ligament and the spinal cord.
The basal parts of the neural arches of the two sides are united
dorsally by a thin cartilaginous layer resting on the sheath of the
notochord, but they are not united ventrally with the hæmal arches.
The hæmal processes in the trunk are much more prominent than in
the preceding stage, and their bases are united ventrally by a
tolerably thick layer of cartilage. In the trunk they are continuous
with the so-called ribs of the adult (Plate 41, fig. 70); but in order to
study the nature of these ribs it is necessary to trace the
modifications undergone by the hæmal arches in passing from the
tail to the trunk.
It will be remembered that at an earlier stage the hæmal arches in
the region of the tail-fin were fully formed, and that through the
anterior part of the caudal region the hæmal processes were far
advanced in development, and just in front of the caudal fin had
actually met below the caudal vein.
The mode of development of the hæmal arches in the tail as
unjointed cartilaginous bars investing the caudal arteries and veins is
so similar to that of the caudal hæmal arches of Elasmobranchii, that
it appears to us impossible to doubt their identity in the two
groups[522]
.](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-40-320.jpg)
![In support of this statement we may point: (1) To the structure of
the sheath of the notochord; (2) to the formation of the greater part
of the bodies of the vertebræ from ossification in membrane around
the notochord; (3) to the early biconcave form of the vertebræ, only
masked at a later period by the development of intervertebral
cartilages; (4) to the character of the neural arches.
This latter feature will be made very clear if the reader will compare
our figures of the sections of later vertebræ (Plate 42, fig. 78) with
Götte's[523]
figure of the section of the vertebra of a Pike (Plate 7, fig.
1). In Götte's figure there are shewn similar intercalated pieces of
cartilage to those which we have found, and similar cartilaginous
dorsal processes of the vertebræ. Thus we are justified in holding
that whether or no the opisthocœlous form of the vertebræ of
Lepidosteus is a commencement of a type of vertebræ inherited by
the higher forms, yet in any case the vertebræ are essentially built
on the type which has become inherited by the Teleostei from the
bony Ganoids.
Part III.—The ribs of Fishes.
The nature and homologies of the ribs of Fishes have long been a
matter of controversy; but the subject has recently been brought
forward in the important memoirs of Götte[524]
on the Vertebrate
skeleton. The alternatives usually adopted are, roughly speaking,
these:—Either the hæmal arches of the tail are homologous
throughout the piscine series, while the ribs of Ganoids and Teleostei
are not homologous with those of Elasmobranchii; or the ribs are
homologous in all the piscine groups, and the hæmal arches in the
tail are differently formed in the different types. Götte has brought
forward a great body of evidence in favour of the first view; while
Gegenbaur[525] may be regarded as more especially the champion of
the second view.
One of us held in a recent publication[526] that the question was not
yet settled, though the view that the ribs are homologous](https://image.slidesharecdn.com/38098-250404070630-2db59e93/85/Mixed-Phase-Clouds-Observations-and-Modeling-eBook-PDF-51-320.jpg)
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- 6. MIXED-PHASE CLOUDS Observations and Modeling Edited by CONSTANTIN ANDRONACHE
- 7. Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States © 2018 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-810549-8 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Candice Janco Acquisition Editor: Laura S. Kelleher Editorial Project Manager: Tasha Frank Production Project Manager: Anitha Sivaraj Cover Designer: Christian J. Bilbow Typeset by SPi Global, India
- 8. CONTRIBUTORS Andrew S. Ackerman National Aeronautics and Space Administration, Goddard Institute for Space Studies, New York, NY, United States Constantin Andronache Boston College, Chestnut Hill, MA, United States Joseph Finlon University of Illinois at Urbana-Champaign, Urbana, IL, United States Jeffrey French University of Wyoming, Laramie, WY, United States Ann M. Fridlind National Aeronautics and Space Administration, Goddard Institute for Space Studies, New York, NY, United States Kalli Furtado Met Office, Exeter, United Kingdom Dennis L. Hartmann University of Washington, Seattle, WA, United States Robert Jackson Argonne National Laboratory, Environmental Sciences Division, Lemont, IL, United States Olivier Jourdan Universit e Clermont Auvergne, Clermont-Ferrand; CNRS, Aubière, France Daniel T. McCoy University of Leeds, Leeds, United Kingdom Steven D. Miller Colorado State University, Fort Collins, CO, United States Guillaume Mioche Universit e Clermont Auvergne, Clermont-Ferrand; CNRS, Aubière, France Yoo-Jeong Noh Colorado State University, Fort Collins, CO, United States Trude Storelvmo Yale University, New Haven, CT, United States ix
- 9. Ivy Tan Yale University, New Haven, CT, United States Thomas F. Whale University of Leeds, Leeds, United Kingdom Mark D. Zelinka Lawrence Livermore National Laboratory, Livermore, CA, United States x Contributors
- 10. PREFACE The objective of this book is to present a series of advanced research topics on mixed- phase clouds. The motivation of this project is the recognized important role clouds play in weather and climate. Clouds influence the atmospheric radiative balance and hydro- logical cycle of the Earth. Reducing uncertainties in weather forecasting and climate pro- jections requires accurate cloud observations and improved representation in numerical cloud models. In this effort to better understand the role of cloud systems, the mixed- phase clouds present particular challenges, which are illustrated in this book. The book has two parts, covering a wide range of topics. The first part, “Observa- tions,” contains articles on cloud microphysics, in situ and ground-based observations, passive and active satellite measurements, and synergistic use of aircraft data with space- borne measurements. The second part, “Modeling,” covers numerical modeling using large eddy simulations to analyze Arctic mixed-phase clouds, and global climate models to address cloud feedbacks and climate sensitivity to mixed-phase cloud characteristics. It is my hope that this book will give some indication of the enormous power and future potential of increasing refined observation techniques and numerical modeling at mul- tiple scales to solve the complex problems of the role of cloud systems in Earth Sciences. The publication of this book would not have been possible without the help, interest, and enthusiasm of the contributing authors. I would like to thank all of the authors and their supporting institutions for making this project possible. I am particularly grateful to Ann Fridlind, Michael Folmer, Daniel McCoy, Ivy Tan, and Michael Tjernstr€ om who offered many useful suggestions during the review process. Finally, it is a great pleasure to acknowledge Candice Janco, Laura Kelleher, Louisa Hutchins, Tasha Frank, Anitha Sivaraj, and Anita Mercy Vethakkan from Elsevier for their willing, dedicated, and con- tinuous help during the project. Constantin Andronache Boston Massachusetts xi
- 11. CHAPTER 1 Introduction Constantin Andronache Boston College, Chestnut Hill, MA, United States Contents 1. Observations 2 2. Modeling 5 3. Concluding Remarks 7 Acknowledgments 7 References 7 Clouds have a significant influence on the atmospheric radiation balance and hydrolog- ical cycle. By interacting with incoming shortwave radiation and outgoing longwave radiation, clouds impact the energy budget of the Earth. They also have an important role in the Earth’s hydrological cycle by affecting water transport and precipitation (Gettelman and Sherwood, 2016). The interaction of clouds with atmospheric radiation depends on hydrometeor phase, size, and shape. Under favorable humidity conditions, the cloud phase is determined largely by the temperature, condensation nuclei, and ice nuclei in the atmosphere. When the cloud temperature is above 0°C, clouds are formed of liquid water droplets. Ice clouds consist of ice crystals and can be found at temperatures well below 0°C. In some clouds, supercooled liquid droplets coexist with ice crystals, most frequently at temperatures from 35°C to 0°C. These are mixed-phase clouds, which are particularly difficult to observe and describe in numerical weather prediction (NWP) and climate models. Mixed-phase clouds cover a large area of the Earth’s surface, and are often persistent, with a liquid layer on top of ice clouds. Many observations have documented the pres- ence of these clouds in all regions of the world and in all seasons (Shupe et al., 2008). They tend to be more frequent at mid- and high-latitudes, where temperatures are favorable to the formation and persistence of supercooled liquid clouds. Global clima- tology is available from CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Sat- ellite Observations (CALIPSO) data, accumulated in recent years (Stephens et al., 2002; Winker et al., 2009; Zhang et al., 2010). Earlier observations, based on aircraft in situ measurements, detected single layer mixed-phase clouds characterized by a layer of supercooled liquid droplets at the top of an ice cloud (Rauber and Tokay, 1991). Over the last decades, in situ observations using instrumented aircraft (Baumgardner et al., 2011), have provided very detailed insights in cloud microphysics and dynamical 1 Mixed-Phase Clouds © 2018 Elsevier Inc. https://doi.org/10.1016/B978-0-12-810549-8.00001-5 All rights reserved.
- 12. conditions that form and maintain these clouds. Such data are essential for calibration of ground-based and spaceborne remote sensing instruments, as well as for the validation of numerical models. Given the importance of mixed-phase clouds in a number of applications, such as the prediction and prevention of aircraft icing, weather modification, and improvement of NWP and climate projections, a series of research programs have contributed to rapid progress in these areas. Selected results are illustrated in this volume, accompanied by references to the most recent studies. The chapters of this book present research on var- ious aspects of mixed-phase clouds, from cloud microphysics to GCM simulations. Chapters 2–6 focus mainly on observational aspects, while Chapters 7–10 illustrate modeling work from small scales using LES to a global scale using GCMs. The next sec- tions give a short description of each chapter. 1. OBSERVATIONS Chapter 2 discusses the relevance of ice nucleation to mixed-phase clouds, and current research on ice nuclei particles (INPs) in the atmosphere. The existence of mixed-phase clouds is possible because liquid water droplets can exist in a supercooled state at tem- peratures as low as 38°C. For lower temperatures, in the absence of INPs, the process of homogeneous ice nucleation can start. The coexistence of liquid water droplets and ice particles in mixed-phase clouds requires specific microphysical and dynamical con- ditions. When a cloud consisting of supercooled liquid water droplets evolves to a state containing some ice crystals, the process of ice nucleation is involved. Despite decades of research, the process of heterogeneous ice nucleation is not sufficiently known (Phillips et al., 2008, 2013; DeMott et al., 2011; Atkinson et al., 2013). A better characterization of the heterogeneous ice nucleation process is needed for the understanding of mixed-phase clouds. This chapter reviews a series of topics relevant for the study of mixed-phase clouds. First, the modes of heterogeneous ice nucleation are described, with a focus on deposition ice nucleation and freezing ice nucleation. Second, the ice nucleation in the atmosphere—particularly in mixed-phase clouds—is summarized and discussed. Third, the experimental methods for examining ice nucleation are presented with a focus on wet and dry dispersion methods. Fourth, the nucleation theory is concisely explained in both homogeneous and heterogeneous cases. Fifth, the properties of good hetero- geneous ice nucleators are discussed, including the direct measurement of INP concen- tration in the atmosphere. This information on direct measurements is particularly important for (a) providing atmospheric model input data, and (b) allowing comparisons between models and observations, thus contributing to the understanding of the ice nucleation processes in the atmosphere. Chapter 3 introduces a method for the detection of liquid-top mixed-phase (LTMP) clouds from satellite passive radiometer observations. While in situ measurements of 2 Mixed-Phase Clouds
- 13. mixed-phase clouds provide detailed information for these clouds, such observations are limited and insufficient for many applications. Satellite remote-sensing techniques are efficient for the continuous monitoring and characterization of mixed-phase clouds. Active satellite sensor measurements, such as CloudSat and CALIPSO have the capability to observe detailed vertical structures of mixed-phase clouds. Nevertheless, they are lim- ited to a spatial domain along the satellite path (Stephens et al., 2002; Winker et al., 2009) and have limited applicability for some short-term purposes. Thus, there is great interest in developing methods for mixed-phase clouds detection using passive radiometry. If adequate methods are developed, satellite remote sensing will provide an ideal venue for observing the global distribution of mixed-phase clouds and the detailed structures such as LTMP clouds. This chapter introduces a method of daytime detection of LTMP clouds from passive radiometer observations, which utilizes reflected sunlight in narrow bands at 1.6 and 2.25 μm to probe below liquid-topped clouds. The basis of the algorithm is established on differential absorption properties of liquid and ice particles and accounts for varying sun/sensor geometry and cloud optical properties (Miller et al., 2014). The algorithm has been applied to the Visible/Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership VIIRS/S-NPP and Himawari-8 Advanced Himawari Imager (Himawari-8 AHI). The measurements with the active sen- sors from CloudSat and CALIPSO were used for evaluation. The results showed that the algorithm has potential to distinguish LTMP clouds under a wide range of conditions, with possible practical applications for the aviation community. Chapter 4 illustrates some of the problems associated with the microphysical proper- ties of convectively forced mixed-phase clouds. Field experiments are conducted using aircraft with particle measurement probes to obtain direct observations of the microphys- ical properties of clouds. Such experiments have been carried out to study various types of cloud systems, including supercooled clouds and mixed-phase clouds. One particular subset of these clouds is the convectively forced mixed-phase clouds. Analysis of obser- vations based on retrievals from CloudSat, CALIPSO, and Moderate Resolution Imag- ing Spectroradiometer (MODIS) show that about 30%–60% of precipitating clouds in the mid- and high-latitudes contain mixed-phase (M€ ulmenst€ adt et al., 2015). In this chapter, authors describe in detail the methodology used in aircraft campaigns, what quantities are typically measured, the importance of particle size distribution (PSD) of hydrometeors, and its moments. The primary in situ measurement methods reviewed include bulk measurements, single particle probes, and imaging probes, with references to recent field campaigns ( Jackson et al., 2012, 2014; Jackson and McFarquhar, 2014). Examples of observations made during the COnvective Precipitation Experiment (COPE) in southwest England during summer 2013 are presented, with a detailed anal- ysis of liquid water content (LWC), ice water content (IWC), and PSD characterization. In general, the microphysical properties of convective clouds can be widely variable due to numerous factors that include temperature, position in the cloud, vertical velocity, 3 Introduction
- 14. strength of entrainment, and the amount of cloud condensation nuclei loaded into the cloud. The study illustrates that determining IWC from the airborne measurement is much more challenging than determining LWC. Therefore, reducing the uncertainty in IWC from airborne cloud microphysical measurements remains an important research priority. Chapter 5 provides an overview of the characterization of mixed-phase clouds from field campaigns and ground-based networks. Earlier field campaigns focused on measure- ments of the microphysical and dynamical conditions of mixed-phase cloud formation and evolution (Rauber and Tokay, 1991; Heymsfield et al., 1991; Heymsfield and Miloshevich, 1993). These studies contributed to solving problems such as aircraft icing and cloud seeding for weather modification. In situ aircraft measurements documented the presence of mixed-phase clouds with a layer of supercooled liquid water on the top of an ice cloud. The US Department of Energy (DOE) Atmospheric Radiation Measure- ment (ARM) program and its focus on the role of clouds in the climate system facilitated many field missions. Some were directed to observations in Arctic regions, aiming to establish a permanent observational station in Barrow, Alaska (Verlinde et al., 2016). Advances in ground-based remote sensing capabilities developed by the ARM program, aided by field campaigns, produced accurate methods to observe atmospheric processes related to water vapor, aerosol, clouds, and radiation. The ability to detect and charac- terize mixed-phase clouds at ARM sites provided the basis for developing additional observation stations in other parts of the world. One significant development in Europe was the Cloudnet program, which established a standard set of ground-based remote sensing instruments capable of providing cloud parameters that can be compared with current operational NWP models (Illingworth et al., 2007). Developments following the Cloudnet program and the expansion of ARM capabilities and collaborations have resulted in a more comprehensive approach for monitoring cloud systems— including mixed-phase clouds—at a variety of sites, enabling the evaluation and improvement of high-resolution numerical models (Haeffelin et al., 2016). Chapter 6 focuses on the characterization of mixed-phase clouds in the Arctic region, using aircraft in situ measurements and satellite observations. Data from the CALIPSO and CloudSat satellites are used to determine the frequency of mixed-phase clouds. Results show that mixed-phase clouds exhibit a frequent and nearly constant presence in the Atlantic side of the Arctic region. In contrast, the Pacific side of the Arctic region has a distinct seasonal variability, with mixed-phase clouds less frequent in winter and spring and more frequent in summer and fall. The vertical distribution of mixed-phase clouds showed that generally, they are present below 3 km, except in summer when these clouds are frequently observed at mid-altitudes (3–6 km). Results indicate that the North Atlantic Ocean and the melting of sea ice influence the spatial, vertical, and seasonal var- iability of mixed-phase clouds (Mioche et al., 2015, 2017). The microphysical and optical properties of the ice crystals and liquid droplets within mixed-phase clouds and the 4 Mixed-Phase Clouds
- 15. associated formation and growth processes responsible for the cloud life cycle are eval- uated based on in situ airborne observations. Lastly, the authors show that the coupling of in situ mixed-phase clouds airborne measurements with the collocated satellite active remote sensing from CloudSat radar and CALIOP lidar measurements are useful in val- idating remote sensing observations. 2. MODELING Chapter 7 provides an overview of numerical simulations of mixed-phase boundary layer clouds using large eddy simulation (LES) modeling. Atmospheric turbulent mixing characterizes boundary layer clouds, and the LES modeling has been extensively used to represent the coupling between dynamical and mixed-phase microphysical processes. Many detailed LES and intercomparison studies have been based on specific cloud sys- tems observed during field campaigns (McFarquhar et al., 2007; Fridlind et al., 2007, 2012; Morrison et al., 2011). The focus of this chapter is mainly on modeling results from the three major field campaigns on which intercomparison studies have been based: the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment- Arctic Cloud Experiment (FIRE-ACE)/Surface Heat Budget in the Arctic (SHEBA) campaign (Curry et al., 2000), the Mixed-Phase Arctic Cloud Experiment (M-PACE) (Verlinde et al., 2007), and the Indirect and Semi-Direct Aerosol Campaign (ISDAC) (McFarquhar et al., 2011). The chapter presents detailed results from each case study and discusses outstanding questions about fundamental microphysical processes of Arctic mixed-phase clouds. Chapter 8 presents efforts toward a parametrization of mixed-phase clouds in general circulation models. Observations show that mid- and high-latitude mixed-phase clouds have a prolonged existence, considerably longer than most models predict. A series of simplified physical models and LES simulations have been applied to data from aircraft observations to understand the factors that lead to the longevity of mixed-phase clouds. The results from many case studies indicate that the persistence of mixed-phase condi- tions is the result of the competition between small-scale turbulent air motions and ice microphysical processes (Korolev and Field, 2008; Hill et al., 2014; Field et al., 2014; Furtado et al., 2016). Under certain situations, this competition can sustain a steady state in which water saturated conditions are maintained for an extended period of time in a constant fraction of the cloud volume. This chapter examines previous work on under- standing this mechanism and explains how it can be elaborated into a parametrization of mixed-phase clouds. The parametrization is constructed on exact, steady state solutions for the statistics of supersaturation variations in a turbulent cloud layer, from which expressions for the liquid-cloud properties can be obtained. The chapter reviews the implementation of the parametrization in a general circulation model. It has been shown to correct the representation of Arctic stratus, compared to in situ observations, and 5 Introduction
- 16. improve the distribution of liquid water at high latitudes. Some important consequences of these enhancements are the reduction in the recognized radiative biases over the Southern Ocean and improvement of the sea surface temperatures in fully coupled cli- mate simulations. Chapter 9 introduces and examines cloud feedback in the climate system. The reflected shortwave (SW) radiation by the oceanic boundary layer (BL) clouds leads to a negative cloud radiative effect (CRE) that strongly affects the Earth’s radiative balance. The response of the BL clouds to climate warming represents a cloud feedback that is highly uncertain in current global climate models. This situation impacts the uncertainty in the estimation of equilibrium climate sensitivity (ECS), defined as the change in the equilibrated surface temperature response to a doubling of atmospheric CO2 concentra- tions. This chapter considers cloud feedback, with a focus on the mid- and high-latitudes where cloud albedo increases with warming, as simulated by global climate models. In these regions, the increase in cloud albedo appears to be caused by mixed-phase clouds transitioning from a more ice-dominated to a more liquid-dominated state (McCoy et al., 2014, 2015, 2016). The chapter discusses problems in constraining mixed-phase clouds in global climate models due to: (a) uncertainties in ice nucleation—a fundamental micro- physical process in mixed-phase clouds formation, and (b) current difficulties in measur- ing the cloud ice mass. Another feature of global climate models is that they use a parameterization of mixed-phase clouds. A frequent approach is to use a phase partition with temperature based on aircraft measurements. One serious limitation of this method is that it cannot account for the regional variability of ice nuclei (IN) (DeMott et al., 2011). Comparisons with satellite data suggest that this behavior appears to be, at least to some extent, due to an inability to maintain supercooled liquid water at sufficiently low temperatures in current global climate models. Chapter 10 addresses the impact of mixed-phase clouds’ supercooled liquid fraction (SLF) on ECS. The ECS is a measure of the ultimate response of the climate system to doubled atmospheric CO2 concentrations. Recent work involving GCM simulations aimed to determine ECS due to changes in the cloud system in a warming climate. This chapter examines the impact of mixed-phase clouds SLF on ECS using a series of coupled climate simulations constrained by satellite observations. It follows a series of recent studies on mixed-phase cloud feedback as determined by GCM simulations (Storelvmo et al., 2015; Tan and Storelvmo, 2016; Tan et al., 2016; Zelinka et al., 2012a,b). This study presents non-cloud feedbacks (Planck, water vapor, lapse rate, and albedo) and cloud feedbacks (cloud optical depth, height, and amount). The cloud phase feedback is a subcategory within the cloud optical depth feedback. It relates to how the repartitioning of cloud liquid droplets and ice crystals affects the reflectivity of mixed-phase clouds. Results suggest that cloud thermodynamic phase plays a significant role in the SW optical depth feedback in the extratropical regions, and ultimately influ- ences climate change. 6 Mixed-Phase Clouds
- 17. 3. CONCLUDING REMARKS The recent research on mixed-phase clouds presented in this volume, as well as the selected references for each chapter, provide an overview of current efforts to appreciate cloud systems and their role in weather and climate. Understanding the role of clouds in the atmosphere is increasingly imperative for applications such as short-term weather forecast, prediction and prevention of aircraft icing, weather modification, assessment of the effects of cloud phase partition on climate models, and accurate climate projections. In response to these challenges, there is a constant need to refine atmospheric observation techniques and numerical models. These efforts are sustained by many evolving research programs and by a vibrant community of scientists. The book “Mixed-phase Clouds: Observations and Modeling” provides the essential information to help readers under- stand the current status of observations, simulations, and applications of mixed-phase clouds, and their implications for weather and climate. ACKNOWLEDGMENTS I want to express my sincere gratitude to all of the authors and reviewers who contributed to this volume. REFERENCES Atkinson, J.D., Murray, B.J., Woodhouse, M.T., Whale, T.F., Baustian, K.J., Carslaw, K.S., Dobbie, S., O’Sullivan, D., Malkin, T.L., 2013. The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds. Nature 498, 355–358. Baumgardner, D., Brenguier, J.-L., Bucholtz, A., Coe, H., DeMott, P., Garrett, T.J., Gayet, J.F., Hermann, M., Heymsfield, A., Korolev, A., Kramer, M., Petzold, A., Strapp, W., Pilewskie, P., Taylor, J., Twohy, C., Wendisch, M., Bachalo, W., Chuang, P., 2011. Airborne instruments to mea- sure atmospheric aerosol particles, clouds and radiation: a cook’s tour of mature and emerging technol- ogy. Atmos. Res. 102 (1-2), 10–29. https://doi.org/10.1016/j.atmosres.2011.06.021. Curry, J.A., Hobbs, P.V., King, M.D., Randall, D., Minnis, P., Isaac, G.A., Pinto, J.O., Uttal, T., Bucholtz, A., Cripe, D., Gerber, H., Fairall, C.W., Garrett, T.J., Hudson, J., Intrieri, J., Jakob, C., Jensen, T., Lawson, P., Marcotte, D., Nguyen, L., Pilewskie, P., Rangno, A., Rogers, D.C., Strawbridge, K.B., Valero, F.P.J., Williams, A.G., Wylie, D., 2000. FIRE arctic clouds experiment. Bull. Am. Meteorol. Soc. 81 (1), 5–29. DeMott, P.J., Mohler, O., Stetzer, O., Vali, G., Levin, Z., Petters, M.D., Murakami, M., Leisner, T., Bundke, U., Klein, H., Kanji, Z.A., Cotton, R., Jones, H., Benz, S., Brinkmann, M., Rzesanke, D., Saatho, H., Nicolet, M., Saito, A., Nillius, B., Bingemer, H., Abbatt, J., Ardon, K., Ganor, E., Georgakopoulos, D.G., Saunders, C., 2011. Resurgence in ice nuclei measurement research. Bull. Am. Meteorol. Soc. 92 (12), 1623–1635. https://doi.org/10.1175/2011BAMS3119.1. Field, P.R., Hill, A., Furtado, K., Korolev, A., 2014. Mixed phase clouds in a turbulent environment. Part 2: analytic treatment. Q. J. Roy. Meteor. Soc. 21, 2651–2663. https://doi.org/10.1002/qj.2175. Fridlind, A.M., Ackerman, A.S., McFarquhar, G.M., Zhang, G., Poellot, M.R., DeMott, P.J., Prenni, A.J., Heymsfield, A.J., 2007. Ice properties of single-layer stratocumulus during the Mixed- Phase Arctic Cloud Experiment: 2. Model results. J. Geophys. Res. 112 (D24), D24202. https://doi. org/10.1029/2007JD008646. Fridlind, A.M., van Diedenhoven, B., Ackerman, A.S., Avramov, A., Mrowiec, A., Morrison, H., Zuidema, P., Shupe, M.D., 2012. A FIRE-ACE/SHEBA case study of mixed-phase Arctic boundary 7 Introduction
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- 19. Miller, S.D., Noh, Y.J., Heidinger, A.K., 2014. Liquid-top mixed-phase cloud detection from shortwave- infrared satellite radiometer observations: a physical basis. J. Geophys. Res. 119. https://doi.org/ 10.1002/2013JD021262. Mioche, G., Jourdan, O., Ceccaldi, M., Delanoë, J., 2015. Variability of mixed-phase clouds in the Arctic with a focus on the Svalbard region: a study based on spaceborne active remote sensing. Atmos. Chem. Phys. 15, 2445–2461. https://doi.org/10.5194/acp-15-2445-2015. Mioche, G., Jourdan, O., Delanoë, J., Gourbeyre, C., Febvre, G., Dupuy, R., Szczap, F., Schwarzenboeck, A., Gayet, J.-F., 2017. Characterization of Arctic mixed-phase cloud properties at small scale and coupling with satellite remote sensing. Atmos. Chem. Phys. Discuss, 1–52. https:// doi.org/10.5194/acp-2017-93. Morrison, H., Zuidema, P., Ackerman, A.S., Avramov, A., De Boer, G., Fan, J., Fridlind, A.M., Hashino, T., Harrington, J.Y., Luo, Y., Ovchinnikov, M., Shipway, B., 2011. Intercomparison of cloud model simulations of Arctic mixed-phase boundary layer clouds observed during SHEBA/ FIRE-ACE. J. Adv. Model. Earth Syst. 3, 1–23. https://doi.org/10.1029/2011MS000066. M€ ulmenst€ adt, J., Sourdeval, O., Delanoë, J., Quaas, J., 2015. Frequency of occurrence of rain from liquid-, mixed-, and ice-phase clouds derived from A-Train satellite retrievals. Geophys. Res. Lett. 42, 6502–6509. https://doi.org/10.1002/2015GL064604. Phillips, V.T.J., DeMott, P.J., Andronache, C., 2008. An empirical parameterization of heterogeneous ice nucleation for multiple chemical species of aerosol. J. Atmos. Sci. 65 (9), 2757–2783. Phillips, V.T.J., DeMott, P.J., Andronache, C., Pratt, K., Prather, K.A., Subramanian, R., Twohy, C., 2013. Improvements to an empirical parameterization of heterogeneous ice nucleation and its compar- ison with observations. J. Atmos. Sci. 70, 378–409. Rauber, R.M., Tokay, A., 1991. An explanation for the existence of supercooled water at the tops of cold clouds. J. Atmos. Sci. 48, 1005–1023. Shupe, M., et al., 2008. A focus on mixed-phase clouds: the status of ground-based observational methods. Bull. Am. Meteorol. Soc. 87, 1549–1562. Stephens, G.K., et al., 2002. The CLOUDSAT Mission and the A-Train—a new dimension of space-based observations of clouds and precipitation. Bull. Am. Meteorol. Soc. 83, 1771–1790. Storelvmo, T., Tan, I., Korolev, A.V., 2015. Cloud phase changes induced by co2 warming—a powerful yet poorly constrained cloud-climate feedback. Curr. Clim. Change Rep. 1 (4), 288–296. Tan, I., Storelvmo, T., 2016. Sensitivity study on the influence of cloud microphysical parameters on mixed-phase cloud thermodynamic phase partitioning in cam5. J. Atmos. Sci. 73 (2), 709–728. Tan, I., Storelvmo, T., Zelinka, M., 2016. Observational constraints on mixed-phase clouds imply higher climate sensitivity. Science 352. https://doi.org/10.1126/science/aad530. Verlinde, J., et al., 2007. The mixed-phase arctic cloud experiment. Bull. Am. Meteorol. Soc. 88, 205–221. Verlinde, J., Zak, B., Shupe, M.D., Ivey, M., Stamnes, K., 2016. The ARM North Slope of Alaska (NSA) sites. The Atmospheric Radiation Measurement (ARM) program: the first 20 years. In: Meteorological Monographs. 57. American Meteorological Society. https://doi.org/10.1175/AMSMONOGRAPHS-D- 15-0023.1. Winker, D.M., Vaughan, M.A., Omar, A.H., Hu, Y., Powell, K.A., Liu, Z., Hunt, W.H., Young, S.A., 2009. Overview of the CALIPSO Mission and CALIOP data processing algorithms. J. Atmos. Ocean. Technol. 26, 2310–2323. https://doi.org/10.1175/2009JTECHA1281.1. Zelinka, M.D., Klein, S.A., Hartmann, D.L., 2012a. Computing and partitioning cloud feedbacks using cloud property histograms. Part i: cloud radiative kernels. J. Clim. 25 (11), 3715–3735. Zelinka, M.D., Klein, S.A., Hartmann, D.L., 2012b. Computing and partitioning cloud feedbacks using cloud property histograms. Part ii: attribution to changes in cloud amount, altitude, and optical depth. J. Clim. 25 (11), 3736–3754. Zhang, D., Wang, Z., Liu, D., 2010. A global view of mid-level liquid-layer topped stratiform cloud dis- tribution and phase partition from CALIPSO and CloudSat measurements. J. Geophys. Res. 115, D00H13. https://doi.org/10.1029/2009JD012143. 9 Introduction
- 20. Discovering Diverse Content Through Random Scribd Documents
- 21. two lobes, the lobi inferiores (Plate 37, fig. 46E, l.in.), which are continued posteriorly into the crura cerebri. The pituitary body[516] (Plate 37, figs. 44, 45, 46E, pt.) is small, not divided into lobes, and provided with a very minute lumen. In front of the infundibulum is the optic chiasma (Plate 37, fig. 46D, op.ch.), which is developed very early. It is, as stated by Müller, a true chiasma. The mid-brain (Plate 37, figs. 44 and 45, m.b.) is large, and consists in both stages of (1) a thickened floor forming the crura cerebri, the central canal of which constitutes the iter a tertio ad quartum ventriculum; and (2) the optic lobes (Plate 37, figs. 46E, F, G, op.l.) above, each of which is provided with a cavity continuous with the median iter. The optic lobes are separated dorsally and in front by a well-marked median longitudinal groove. Posteriorly they largely overlap the cerebellum. In the anterior part of the optic lobes, at the point where the iter joins the third ventricle, there may be seen slight projections of the floor into the lumen of the optic lobes (Plate 37, fig. 46E). These masses probably become in the adult the more conspicuous prominences of the floor of the ventricles of the optic lobes, which we regard as homologous with the tori semicirculares of the brain of the Teleostei. The hind-brain is formed of the usual divisions, viz.: cerebellum and medulla oblongata (Plate 37, figs. 44 and 45, cb., md.). The former constitutes a bilobed projection of the roof of the hind-brain. Only a small portion of it is during these stages left uncovered by the optic lobes, but the major part extends forwards for a considerable distance under the optic lobes, as shewn in the transverse sections (Plate 37, figs. 46F and G, cb.); and its two lobes, each with a prolongation of its cavity, are continued forwards beyond the opening of the iter into the fourth ventricle. It is probable that the anterior horns of the cerebellum are equivalent to the prolongations of the cerebellum into the central
- 22. cavity of the optic lobes of Teleostei, which are continuous with the so-called fornix of Göttsche. III. Comparison of the larval and adult brain of Lepidosteus, together with some observations on the systematic value of the characters of the Ganoid brain. The brain of the older of the two larvæ, which we have described, sufficiently resembles in most of its features that of the adult to render material assistance in the interpretation of certain of the parts of the latter. It will be remembered that in the adult brain the parts usually held to be olfactory lobes were described as the anterior cerebral lobes. The grounds for this will be apparent by a comparison of the cerebrum of the larva and adult. In the larva the cerebrum is formed of (1) an unpaired basal portion with a thin roof, and (2) of a pair of anterior lobes, with small olfactory bulbs at their free extremities. The basal portion in the larva clearly corresponds in the adult with the basal portion, together with the two posterior cerebral lobes, which are merely special outgrowths of the dorsal edge of the primitive basal portion. The pair of anterior lobes have exactly the same relations in the larva as in the adult, except that in the former the ventricles are prolonged for their whole length instead of being confined to their proximal portions. If, therefore, our identifications of the larval parts of the brain are correct, there can hardly be a question as to our identifications of the parts in the adult. As concerns these identifications, the comparison of the brain of our two larvæ appears conclusive in favour of regarding the anterior lobes as parts of the cerebrum, as distinguished from the olfactory lobes, in that they are clearly derived from the undivided anterior portion of the cerebrum of the younger larva. The comparison of the larval brain with that of the adult again appears to us to leave no doubt that the vesicle attached to the roof of the thalamencephalon in the adult is the same structure as the
- 23. bilobed outgrowth of this roof in the larva; and since there is in addition a well-developed pineal gland in the larva with the usual relations, there can be no ground for identifying the vesicle in the adult with the pineal gland. Müller, in his often quoted memoir (No. 13), states that the brains of Ganoids are peculiar and distinct from those both of Teleostei and Elasmobranchii; but in addition to pointing out that the optic nerves form a chiasma he does not particularly mention the features, to which he alludes in general terms. More recently Wilder (No. 15) has returned to this subject; and though, as we have already had occasion to point out, we cannot accept all his identifications of the parts of the Ganoid brain, yet he has called attention to certain characteristic features of the cerebrum which have an undoubted systematic value. The distinctive characters of the Ganoid brain are, in our opinion, (1) the great elongation of the region of the thalamencephalon; and (2) the unpaired condition of the posterior part of the cerebrum, and the presence of so thin a roof to the ventricle of this part as to cause it to appear open above. The immense length of the region of the thalamencephalon is a feature in the Ganoid brain which must at once strike any one who examines figures of the brains of Chondrostei, Polypterus, or Amia. It is less striking in the adult Lepidosteus, though here also we have shewn that the thalamencephalon is really very greatly developed; but in the larva of Lepidosteus this feature is still better marked, so that the brain of the larva may be described as being more characteristically Ganoid than that of the adult. The presence of a largely developed thalamencephalon at once distinguishes a Ganoid brain from that of a Teleostean Fish, in which the optic thalami are very much reduced; but Lepidosteus shews its Teleostean affinities by a commencing reduction of this part of the brain.
- 24. The large size of the thalamencephalon is also characteristic of the Ganoid brain in comparison with the brain of the Dipnoi; but is not however so very much more marked in the Ganoids than it is in some Elasmobranchii. On the whole, we may consider the retention of a large thalamencephalon as a primitive character. The second feature which we have given as characteristic of the Ganoid brain is essentially that which has been insisted upon by Wilder, though somewhat differently expressed by him. The simplest condition of the cerebrum is that found in the larva of Lepidosteus, where there is an anterior pair of lobes, and an undivided posterior portion with a simple prolongation of the third ventricle, and a very thin roof. The dorsal edges of the posterior portion, adjoining the thin roof, usually become somewhat everted (cf. Wilder), and in Lepidosteus these edges have in the adult a very great development, and form (vide Plate 38, fig. 47A-C, ce´.) two prominent lobes, which we have spoken of as the posterior cerebral lobes. These characters of the cerebrum are perhaps even more distinctive than those of the thalamencephalon. In Teleostei the cerebrum appears to be completely divided into two hemispheres, which are, however, all but solid, the lateral ventricles being only prolonged into their bases. In Dipnoi again there is either (Protopterus, Wiedersheim[517]) a completely separated pair of oval hemispheres, not unlike those of the lower Amphibia, or the oval hemispheres are not completely separated from each other (Ceratodus, Huxley[518], Lepidosiren, Hyrtl[519]); in either case the hemispheres are traversed for the whole length by lateral ventricles which are either completely or nearly completely separated from each other. In Elasmobranchii the cerebrum is an unpaired though bilobed body, but traversed by two completely separated lateral ventricles, and
- 25. without a trace of the peculiar membranous roof found in Ganoids. Not less interesting than the distinguishing characters of the Ganoid brain are those cerebral characters which indicate affinities between Lepidosteus and other groups. The most striking of these are, as might have been anticipated, in the direction of the Teleostei. Although the foremost division of the brain is very dissimilar in the two groups, yet the hind-brain in many Ganoids and the mid-brain also in Lepidosteus approaches closely to the Teleostean type. The most essential feature of the cerebellum in Teleostei is its prolongation forwards into the ventricles of the optic vesicles as the valvula cerebelli. We have already seen that there is a homologous part of the cerebellum in Lepidosteus; Stannius also describes this part in the Sturgeon, but no such part is represented in Müller's figure of the brain of Polypterus, or described by him in the text. The cerebellum is in most Ganoids relatively smaller, and this is even the case with Amia; but the cerebellum of Lepidosteus is hardly less bulky than that of most Teleostei. The presence of tori semicirculares on the floor of the mid-brain of Lepidosteus again undoubtedly indicates its affinities with the Teleostei, and such processes are stated by Stannius to be absent in the Sturgeon, and have not, so far as we are aware, been described in other Ganoids. Lastly we may point to the presence of well- developed lobi inferiores in the brain of Lepidosteus as an undoubted Teleostean character. On the whole, the brain of Lepidosteus, though preserving its true Ganoid characters, approaches more closely to the brain of the Teleostei than that of any other Ganoid, including even Amia. It is not easy to point elsewhere to such marked resemblances of the Ganoid brain, as to the brain of the Teleostei. The division of the cerebrum into anterior and posterior lobes, which is found in Lepidosteus, probably reappears again, as already
- 26. indicated, in the higher Amphibia. The presence of the peculiar vesicle attached to the roof of the thalamencephalon has its parallel in the brain of Protopterus, and as pointing in the same direction a general similarity in the appearance of the brain of Polypterus to that of the Dipnoi may be mentioned. There appears to us to be in no points a close resemblance between the brain of Ganoids and that of Elasmobranchii. [509] The homologies of the olfactory lobes throughout the group of Fishes require further investigation. [510] “Ueb. d. Gehirn des Störs,” Müller's Archiv, 1843, and Lehrbuch d. vergl. Anat. d. Wirbelthiere. Cattie, Archives de Biologie, Vol. III. 1882, has recently described in Acipenser sturio a vesicle on the roof of the thalamencephalon, whose cavity is continuous with the third ventricle. This vesicle is clearly homologous with that in Lepidosteus. (June 28, 1882.) [511] R. Wiedersheim, Morphol. Studien, 1880, p. 71. [512] “On Ceratodus Forsteri,” c., Proc. Zool. Soc. 1876. [513] In Wilder's figure the walls of the cerebellum are represented as much too thin. [514] Vide F. M. Balfour, Comparative Embryology, Vol. II. figs. 248 and 250. [515] Vide F. M. Balfour, Comparative Embryology, Vol. II. pp. 355-6 [the original edition], where it is suggested that this commissure is the homologue of the grey commissure of higher types. [516] We have not been able to work out the early development of the pituitary body as satisfactorily as we could have wished. In Plate 37, fig. 40, there is shewn an invagination of the oral epithelium to form it; in Plate 37, figs. 41 and 42, it is represented in transverse section in two consecutive sections. Anteriorly it is still connected with the oral epithelium (fig. 41), while posteriorly it is free. It is possible that an earlier stage of it is shewn in Plate 36, fig. 35. Were it not for the evidence in other types of its being derived from the epiblast we should be inclined to regard it as hypoblastic in origin. [517] Morphol. Studien, III. Jena, 1880.
- 27. [518] “On Ceratodus Forsteri,” Proc. Zool. Soc. 1876. [519] Lepidosiren paradoxa. Prag. 1845. Sense Organs. Olfactory organ. Development.—The nasal sacks first arise during the late embryonic period in the form of a pair of thickened patches of the nervous layer of the epiblast on the dorsal surface of the front end of the head (Plate 37, fig. 39, ol.). The patches very soon become partially invaginated; and a small cavity is developed between them and the epidermic layer of the epiblast (Plate 37, figs. 42 and 43, ol.). Subsequently, the roof of this space, formed by the epidermic layer of the epiblast, is either broken through or absorbed; and thus open pits, lined entirely by the nervous layer of the epidermis, are formed. We are not acquainted with any description of an exactly similar mode of origin of the olfactory pits, though the process is almost identical with that of the other sense organs. We have not worked out in detail the mode of formation of the double openings of the olfactory pits, but there can be but little doubt that it is caused by the division of the single opening into two. The olfactory nerve is formed very early (Plate 37, fig. 39, I), and, as Marshall has found in Aves and Elasmobranchii, it arises at a stage prior to the first differentiation of an olfactory bulb as a special lobe of the brain. The Eye. Anatomy.—We have not made a careful histological examination of the eye of Lepidosteus, which in our specimens was not sufficiently
- 28. well preserved for such a purpose; but we have found a vascular membrane enveloping the vitreous humour on its retinal aspect, which, so far as we know, is unlike anything which has so far been met with in the eye of any other adult Vertebrate. The membrane itself is placed immediately outside the hyaloid membrane, i.e. on the side of the hyaloid membrane bounding the vitreous humour. It is easily removed from the retina, to which it is only adherent at the entrance of the optic nerve. In both the eyes we examined it also adhered, at one point, to the capsule of the lens, but we could not make out whether this adhesion was natural, or artificially produced by the coagulation of a thin layer of albuminous matter. In one instance, at any rate, the adhesion appeared firmer than could easily be produced artificially. The arrangement of the vessels in the membrane is shewn diagrammatically in Plate 38, fig. 49, while the characteristic form of the capillary plexus is represented in Plate 38, fig. 50. The arterial supply appears to be derived from a vessel perforating the retina close to the optic nerve, and obviously homologous with the artery of the processus falciformis and pecten of Teleostei and Birds, and with the arteria centralis retinæ of Mammals. From this vessel branches diverge and pursue a course towards the periphery. They give off numerous branches, the blood from which enters a capillary plexus (Plate 38, figs. 49 and 50) and is collected again by veins, which pass outwards and finally bend over and fall into (Plate 38, fig. 49) a circular vein (cr.v.) placed at the outer edge of the retina along the insertion of the iris (ir). The terminal branches of some of the main arteries appear also to fall directly into this vein. The membrane supporting the vessels just described is composed of a transparent matrix, in which numerous cells are embedded (Plate 38, fig. 50). Development.—In the account of the first stages of development of Lepidosteus, the mode of formation of the optic cup, the lens, c.,
- 29. have been described (vide Plates 35 and 36, figs. 23, 26, 35). With reference to the later stages in the development of the eye, the only subject with which we propose to deal is the growth of the mesoblastic processes which enter the cavity of the vitreous humour through the choroid slit. Lepidosteus is very remarkable for the great number of mesoblast cells which thus enter the cavity of the vitreous humour, and for the fact that these cells are at first unaccompanied by any vascular structures (Plate 37, fig. 43, v.h). The mesoblast cells are scattered through the vitreous humour, and there can be no doubt that during early larval life, at a period however when the larva is certainly able to see, every histologist would consider the vitreous humour to be a tissue formed of scattered cells, with a large amount of intercellular substance; and the fact that it is so appears to us to demonstrate that Kessler's view of the vitreous humour being a mere transudation is not tenable. In the larva five or six days after hatching, and about 15 millims. in length, the choroid slit is open for its whole length. The edges of the slit near the lens are folded, so as to form a ridge projecting into the cavity of the vitreous humour, while nearer the insertion of the optic nerve they cease to exhibit any such structure. The mesoblast, though it projects between the lips of the ridge near the lens, only extends through the choroid slit into the cavity of the vitreous humour in the neighbourhood of the optic nerve. Here it forms a lamina with a thickened edge, from which scattered cells in the cavity of the vitreous humour seem to radiate. At a slightly later stage than that just described, blood-vessels become developed within the cavity of the vitreous humour, and form the vascular membrane already described in the adult, placed close to the layer of nerve-fibres of the retina, but separated from this layer by the hyaloid membrane (Plate 38, fig. 48, v.sh.). The artery bringing the blood to the above vascular membrane is bound up in the same sheath as the optic nerve, and passes through the choroid slit very close to the optic nerve. Its entrance into the cavity
- 30. of the vitreous humour is shewn in Plate 38, fig. 48 (vs.); its relation to the optic nerve in Plate 37, fig. 46, C and D (vs.). The above sheath has, so far as we know, its nearest analogue in the eye of Alytes, where, however, it is only found in the larva. The reader who will take the trouble to refer to the account of the imperfectly-developed processus falciformis of the Elasmobranch eye in the treatise On Comparative Embryology, by one of us[520] , will not fail to recognize that the folds of the retina at the sides of the choroid slit, and the mesoblastic process passing through this slit, are strikingly similar in Lepidosteus and Elasmobranchii; and that, if we are justified in holding them to be an imperfectly-developed processus falciformis in the one case, we are equally so in the other. Johannes Müller mentions the absence of a processus falciformis as one of the features distinguishing Ganoids and Teleostei. So far as the systematic separation of the two groups is concerned, he is probably perfectly justified in this course; but it is interesting to notice that both in Ganoids and Elasmobranchii we have traces of a structure which undergoes a very special development in the Teleostei, and that the processus falciformis of Teleostei is therefore to be regarded, not as an organ peculiar to them, but as the peculiar modification within the group of a primitive Vertebrate organ. [520] Vol. II. p. 414 [the original edition]. Suctorial Disc. One of the most remarkable organs of the larval Lepidosteus is the suctorial disc, placed at the front end of the head, to which we have made numerous allusions in the first section of this memoir. The external features of the disc have been fully dealt with by Agassiz, and he also explained its function by observations on the
- 31. habits of the larva. We have already quoted (p. 755) a passage from Agassiz' memoir shewing how the young Fishes use the disc to attach themselves firmly to any convenient object. The discs appear in fact to be highly efficient organs of attachment, in that the young Fish can remain suspended by them to the sides of the jar, even after the water has been lowered below the level at which they are attached. The disc is formed two or three days before hatching, and from Agassiz' statements, it appears to come into use immediately the young Fish is liberated from the egg membranes. We have examined the histological structure of the disc at various ages of its growth, and may refer the reader to Plate 34, figs. 11 and 13, and Plate 37, figs. 40 and 44. The result of our examination has been to shew that the disc is provided with a series of papillæ often exhibiting a bilateral arrangement. The papillæ are mainly constituted of highly modified cells of the mucous layer of the epidermis. These cells have the form of elongated columns, the nucleus being placed at the base, and the main mass of the cells being filled with a protoplasmic reticulum. They may probably be regarded as modified mucous cells. In the mesoblast adjoining the suctorial disc there are numerous sinus-like vascular channels. It does not appear probable that the disc has a true sucking action. It is unprovided with muscular elements, and there appears to be no mechanism by which it could act as a sucking organ. We must suppose, therefore, that its adhesive power depends upon the capacity of the cells composing its papillæ to pour out a sticky secretion. Muscular System.
- 32. There is a peculiarity in the muscular system of Lepidosteus, which so far as we know has not been previously noticed. It is that the lateral muscles of each side are not divided, either in the region of the trunk or of the tail, into a dorso-lateral and ventro-lateral division. This peculiarity is equally characteristic of the older larvæ as of the adult, and is shewn in Plate 41, figs. 67, 72, and 73, and Plate 42, figs. 74-76. In the Cyclostomata the lateral muscles are not divided into dorsal and ventral sections; but except in this group such a division has been hitherto considered as invariable amongst Fishes. This character must, without doubt, be held to be the indication of a very primitive arrangement of the muscular system. In the embryos of all Fishes with the usual type of the lateral muscles, the undivided condition of the muscles precedes the divided condition; and in primitive forms such as the Cyclostomata and Amphioxus the embryonic condition is retained, as it is in Lepidosteus. Skeleton. Part I.—Vertebral column and ribs of the adult. A typical vertebra from the trunk of Lepidosteus has the following characters (Plate 42, figs. 80 and 81). The centrum is slightly narrower in the middle than at its two extremities. It articulates with adjacent vertebræ by a convex face in front and a concave face behind, being thus, according to Owen's nomenclature, opisthocœlous. It presents on its under surface a well-marked longitudinal ridge, which in many vertebræ is only united at its two extremities with the main body of the vertebra.
- 33. From the lateral borders of the centrum there project, at a point slightly nearer the front than the hind end, a pair of prominent hæmal processes (h.a.), to the ends of which are articulated the ribs. These processes have a nearly horizontal direction in the greater part of the trunk, though bent downwards in the tail. The neural arches (n.a.) have a somewhat complicated form. They are mainly composed of two vertical plates, the breadth of the basal parts of which is nearly as great as the length of the vertebræ, so that comparatively narrow spaces are left between the neural arches of successive vertebræ for the passage of the spinal nerves. Some little way from its dorsal extremity each neural arch sends a horizontal process inwards, which meets its fellow and so forms a roof for the spinal canal. These processes appear to be confined to the posterior parts of the vertebræ, so that at the front ends of the vertebræ, and in the spaces between them, the neural canal is without an osseous roof. Above the level of this osseous roof there is a narrow passage, bounded laterally by the dorsal extremities of the neural plates. This passage is mainly filled up by a series of cartilaginous elements (Plate 42, figs. 80 and 81, i.c.) (probably fibro-cartilage), which rest upon the roof of the neural canal. Each element is situated intervertebrally, its anterior end being wedged in between the two dorsal processes of the neural arch of the vertebra in front, and its posterior end extending for some distance over the vertebra behind. The successive elements are connected by fibrous tissue, and are continuous dorsally with a fibrous band, known as the ligamentum longitudinale superius (Plate 42, figs. 80 and 81, l.l.), characteristic of Fishes generally, and running continuously for the whole length of the vertebral column. Each of the cartilaginous elements is, as will be afterwards shewn, developed as two independent pieces of cartilage, and might be compared with the dorsal element which usually forms the keystone of the neural arch in Elasmobranchii, were not the latter vertebral instead of intervertebral in position. More or less similar elements are described by Götte in the neural arches of many Teleostei, which also, however, appear to be vertebrally placed, and he has compared
- 34. them and the corresponding elements in the Sturgeon with the Elasmobranch cartilages forming the keystone of the neural arch. Götte does not, however, appear to have distinguished between the cartilaginous elements, and the osseous elements forming the roof of the spinal canal, which are true membrane bones; it is probable that the two are not so clearly separated in other types as in Lepidosteus. The posterior ends of the neural plates of the neural arches are continued into the dorsal processes directed obliquely upwards and backwards, which have been somewhat unfortunately described by Stannius as rib-like projections of the neural arch. The dorsal processes of the two sides do not meet, but between them is placed a median free spinous element, also directed obliquely upwards and backwards, which forms a kind of roof for the groove in which the cartilaginous elements and the ligamentum longitudinale are placed. The vertebræ are wholly formed of a very cellular osseous tissue, in which a distinction between the bases of the neural and hæmal processes and the remainder of the vertebra is not recognizable. The bodies of the vertebræ are, moreover, directly continuous with the neural and hæmal arches. The ribs in the region of the trunk are articulated to the ends of the long hæmal processes. They envelop the body-cavity, their proximal parts being placed immediately outside the peritoneal membrane, along the bases of the intermuscular septa. Their distal ends do not, however, remain close to the peritoneal membrane, but pass outwards along the intermuscular septa till their free ends come into very close proximity with the skin. This peculiarity, which holds good in the adult for all the free ribs, is shewn in one of the anterior ribs of an advanced larva in Plate 41, fig. 72 (rb.). We are not aware that this has been previously noticed, but it appears to us to be a point not without interest in all questions which concern the homology of rib-like structures occupying different positions in relation to the muscles. Its bearings are fully dealt with in the section of this paper devoted to the consideration of the homologies of the ribs in Fishes.
- 35. As regards the behaviour of the ribs in the transitional region between the trunk and the tail, we cannot do better than translate the description given by Gegenbaur of this region (No. 6, p. 411): —“Up to the 34th vertebra the ribs borne by the laterally and posteriorly directed processes present nothing remarkable, though they have gradually become shorter. The ribs of the 35th vertebra exhibit a slight curvature outwards of their free ends, a peculiarity still more marked in the 36th. The last named pair of ribs converge somewhat in their descent backwards so that both ribs decidedly approach before bending outwards. The 37th vertebra is no longer provided with freely terminating ribs, but on the contrary, the same pair of processes which in front was provided with ribs, bears a short forked process as the hæmal arch. The two, up to this point separated ribs, have here formed a hæmal arch by the fusion of their lower ends, which arch is movable just like the ribs, and, like them, is attached to the vertebral column.” In the region of the tail-fin the hæmal arches supporting the caudal fin-rays are very much enlarged. Part II.—Development of the vertebral column and ribs. The first development and early histological changes of the notochord have already been given, and we may take up the history of the vertebral column at a period when the notochord forms a large circular rod, whose cells are already highly vacuolated, while the septa between the vacuoles form a delicate wide-meshed reticulum. Surrounding the notochord is the usual cuticular sheath, which is still thin. The first indications of the future vertebral column are to be found in the formation of a distinct mesoblastic investment of the notochord. On the dorsal aspect of the notochord, the mesoblast forms two ridges, one on each side, which are prolonged upwards so as to meet above the neural canal, for which they form a kind of sheath. On the ventral side of the notochord there are also two ridges, which
- 36. are, however, except on the tail, much less prominent than the dorsal ridges. The changes which next ensue are practically identical with those which take place in Teleostei. Around the cuticular sheath of the notochord there is formed an elastic membrane—the membrana elastica externa. At the same time the basal parts of the dorsal, or as we may perhaps more conveniently call them, the neural ridges of the notochord become enlarged at each intermuscular septum, and the tissue of these enlargements soon becomes converted into cartilage, thus forming a series of independent paired neural processes riding on the membrana elastica externa surrounding the notochord, and extending about two-thirds of the way up the sides of the medullary cord. They are shewn in transverse section in Plate 41, fig. 67 (n.a.), and in a side view in fig. 68 (n.a.). Simultaneously with the neural arches, the hæmal arches also become established, and arise by the formation of similar enlargements of the ventral or hæmal ridges. In the trunk they are very small, but in the region of the tail their condition is very different. At the front end of the anal fin the paired hæmal arches suddenly enlarge and extend ventralwards (Plate 41, fig. 67, h.a.). Each succeeding pair of arches becomes larger than the one in front, and the two elements of each arch first nearly meet below the caudal vein (Plate 41, fig. 67) and finally actually do so, forming in this way a completely closed hæmal canal. At the point where they first meet the permanent caudal fin commences, and here (Plate 41, fig. 68) we find that not only do the hæmal arches meet and coalesce below the caudal vein, but they are actually produced into long spines supporting the fin-rays of the caudal fin, which thus differs from the other fins in being supported by parts of the true vertebral column and not by independently formed elements of the skeleton. Each of the large caudal hæmal arches, including the spine, forms a continuous [TN18] whole, and arises at an earlier period of larval life
- 37. than any other part of the vertebral column. We noticed the first indications of the neural arches in the larva of about a week old, while they are converted into fully formed cartilage in the larva of three weeks. The neural and hæmal arches, resting on the membrana elastica externa, do not at this early stage in the least constrict the notochord. They grow gradually more definite, till the larva is five or six weeks old and about 26 millims. in length, but otherwise for a long time undergo no important changes. During the same period, however, the true sheath of the notochord greatly increases in thickness, and the membrana elastica externa becomes more definite. So far it would be impossible to distinguish the development of the vertebral column of Lepidosteus from that of a Teleostean Fish. Of the stages immediately following we have unfortunately had no examples, but we have been fortunate enough to obtain some young specimens of Lepidosteus[521], which have enabled us to work out with tolerable completeness the remainder of the developmental history of the vertebral column. In the next oldest larva, of about 5.5 centims., the changes which have taken place are already sufficient to differentiate the vertebral column of Lepidosteus from that of a Teleostean, and to shew how certain of the characteristic features of the adult take their origin. In the notochord the most important and striking change consists in the appearance of a series of very well marked vertebral constrictions opposite the insertions of the neural and hæmal arches. The first constrictions of the notochord are thus, as in other Fishes, vertebral; and although, owing to the growth of the intervertebral cartilage, the vertebral constrictions are subsequently replaced by intervertebral constrictions, yet at the same time the primitive occurrence of vertebral constrictions demonstrates that the vertebral column of Lepidosteus is a modification of a type of vertebral column with biconcave vertebræ.
- 38. The structure of the gelatinous body of the notochord has undergone no important change. The sheath, however, exhibits certain features which deserve careful description. In the first place the attention of the observer is at once struck by the fact that, in the vertebral regions, the sheath is much thicker (.014 millims.) than in the intervertebral (.005 millims.), and a careful examination of the sheath in longitudinal sections shews that the thickening is due to the special differentiation of a superficial part (Plate 41, fig. 69, sh.) of the sheath in each vertebral region. This part is somewhat granular as compared to the remainder, especially in longitudinal sections. It forms a cylinder (the wall of which is about .01 millim. thick) in each vertebral region, immediately within the membrana elastica externa. Between it and the gelatinous tissue of the notochord within there is a very thin unmodified portion of the sheath, which is continuous with the thinner intervertebral parts of the sheath. This part of the sheath is faintly, but at the same time distinctly, concentrically striated—a probable indication of concentric fibres. The inner unmodified layer of the sheath has the appearance in transverse sections through the vertebral regions of an inner membrane, and may perhaps be Kölliker's “membrana elastica interna.” We are not aware that any similar modification of the sheath has been described in other forms. The whole sheath is still invested by a very distinct membrana elastica externa (m.el). The changes which have taken place in the parts which form the permanent vertebræ will be best understood from Plate 41, figs. 69- 71. From the transverse section (fig. 70) it will be seen that there are still neural and hæmal arches resting upon the membrana elastica externa; but longitudinal sections (fig. 69) shew that laterally these arches join a cartilaginous tube, embracing the intervertebral regions of the notochord, and continuous from one vertebra to the next.
- 39. It will be convenient to treat separately the neural arches, the hæmal arches with their appendages, and the intervertebral cartilaginous rings. The neural arches, except in the fact of embracing a relatively smaller part of the neural tube than in the earlier stage, do not at first sight appear to have undergone any changes. Viewed from the side, however, in dissected specimens, they are seen to be prolonged upwards so as to unite above with bars of cartilage directed obliquely backwards. An explanation of this appearance is easily found in the sections. The cartilaginous neural arches are invested by a delicate layer of homogeneous bone, developed in the perichondrium, and this bone is prolonged beyond the cartilage and joins a similar osseous investment of the dorsal bars above mentioned. The whole of these parts may, it appears to us, be certainly reckoned as parts of the neural arches, so that at this stage each neural arch consists of: (1) a pair of basal portions resting on the notochord consisting of cartilage invested by bone, (2) of a pair of dorsal cartilaginous bars invested in bone (n.a´.), and (3) of osseous bars connecting (1) and (2). Though, in the absence of the immediately preceding stages, it is not perfectly certain that the dorsal pieces of cartilage are developed independently of the ventral, there appears to us every probability that this is so; and thus the cartilage of each neural arch is developed discontinuously, while the permanent bony neural arch, which commences as a deposit of bone partly in the perichondrium and partly in the intervening membrane, forms a continuous structure. Analogous occurrences have been described by Götte in Teleostei. The dorsal portion of each neural arch becomes what we have called the dorsal process of the adult arch. Between the dorsal processes of the two sides there is placed a median rod of cartilage (Plate 41, fig. 70, i.s.), which in its
- 40. development is wholly independent of the true neural arches, and which constitutes the median spinous element of the adult. In tracing these backwards it becomes obvious that they are homologous with the interspinous elements supporting the dorsal fin, in that they are replaced by these interspinous elements in the region of the dorsal fin, and that the interspinous bones occupy the same position as the median spinous processes. This homology was first pointed out by Götte in the case of the Teleostei. Immediately beneath this rod is placed the longitudinal ligament (Plate 41, fig. 70, l.l.), but there is as yet no trace of a junction between the neural arches of the two sides in the space between the longitudinal ligament and the spinal cord. The basal parts of the neural arches of the two sides are united dorsally by a thin cartilaginous layer resting on the sheath of the notochord, but they are not united ventrally with the hæmal arches. The hæmal processes in the trunk are much more prominent than in the preceding stage, and their bases are united ventrally by a tolerably thick layer of cartilage. In the trunk they are continuous with the so-called ribs of the adult (Plate 41, fig. 70); but in order to study the nature of these ribs it is necessary to trace the modifications undergone by the hæmal arches in passing from the tail to the trunk. It will be remembered that at an earlier stage the hæmal arches in the region of the tail-fin were fully formed, and that through the anterior part of the caudal region the hæmal processes were far advanced in development, and just in front of the caudal fin had actually met below the caudal vein. The mode of development of the hæmal arches in the tail as unjointed cartilaginous bars investing the caudal arteries and veins is so similar to that of the caudal hæmal arches of Elasmobranchii, that it appears to us impossible to doubt their identity in the two groups[522] .
- 41. The changes which have taken place by this stage with reference to the hæmal arches of the tail are not very considerable. In the case of a few more vertebræ the hæmal processes have united into an arch, and the spinous processes of the arches in the region of the caudal fin have grown considerably in length. A more important change is perhaps the commencement of a segmentation of the distal parts of the hæmal arches from the proximal. This process has not, however, as yet resulted in a complete separation of the two, such as we find in the adult. If the hæmal processes are traced forwards (Plate 42, figs. 75 and 76) from the anterior segment where they meet ventrally, it will be found that each hæmal process consists of a basal portion, adjoining the notochord, and a peripheral portion. These two parts are completely continuous, but the line of a future separation is indicated by the structure of the cartilage, though not shewn in our figures. As the true body-cavity of the trunk replaces the obliterated body-cavity of the caudal region, no break of continuity will be found in the structure of the hæmal processes (Plates 41 and 42, figs. 73 and 74), but while the basal portions grow somewhat larger, the peripheral portions gradually elongate and take the form of delicate rods of cartilage extending ventralwards, on each side of the body- cavity, immediately outside the peritoneal membrane, and along the lines of insertion of the intermuscular septa. These rods obviously become the ribs of the adult. As one travels forwards the ribs become continually longer and more important, and though they are at this stage united with the hæmal processes in every part of the trunk, yet they are much more completely separated from these processes in front than behind (Plate 41, fig. 72). In front (Plate 41, fig. 72), each rib (rb.), after continuing its ventral course for some distance, immediately outside the peritoneal membrane, turns outwards, and passes along one of the intermuscular septa till it reaches the epidermis. This feature in the
- 42. position of the ribs is, as has been already pointed out in the anatomical part of this section, characteristic of all the ribs of the adult. It is unfortunate that we have had no specimens shewing the ribs at an earlier stage of development; but it appears hardly open to doubt that the ribs are originally continuous with the hæmal processes, and that the indications of a separation between those two parts at this stage are not due to a secondary fusion, but to a commencing segmentation. It further appears, as Müller, Gegenbaur and others have stated, that the ribs and hæmal processes of the tail are serially homologous structures; but that the view maintained by Götte in his very valuable memoirs on the Vertebrate skeleton is also correct to the effect that the hæmal arches of the tail are homologous throughout the series of Fishes. To this subject we shall return again at the end of the section. Before leaving the hæmal arches it may be mentioned that behind the region of the ventral caudal fin the two hæmal processes merge into one, and form an unpaired knob resting on the ventral side of the notochord, and not perforated by a canal. There are now present well-developed intervertebral rings of cartilage, each of which eventually becomes divided into two parts, and converted into the adjacent faces of the contiguous vertebræ. These rings are united with the neural and hæmal arches of the vertebræ in front and behind. Each ring, as shewn by the transverse section (Plate 41, fig. 71), is not uniformly thick, but exhibits four projections, two dorsal and two ventral. These four projections are continuous with the bases of the neural and hæmal arches of the adjacent vertebræ, and afford presumptive evidence of the derivation of the intervertebral rings from the neural and hæmal arches; in that had they so originated, it would be natural to anticipate the presence of four thickenings
- 43. indicating the four points from which the cartilage had spread, while if the rings had originated independently, it would not be easy to give any explanation of the presence of such thickenings. Gegenbaur (No. 6), from the investigation of a much older larva than that we are now describing, also arrived at the conclusion that the intervertebral cartilages were derived from the neural and hæmal arches; but as doubts have been thrown upon this conclusion by Götte, and as it obviously required further confirmation, we have considered it important to attempt to settle this point. From the description given above, it is clear that we have not, however, been able absolutely to trace the origin of this cartilage, but at the same time we think that we have adduced weighty evidence in corroboration of Gegenbaur's view. As shewn in longitudinal section (Plate 41, fig. 69, iv.r.), the intervertebral rings are thicker in the middle than at the two ends. In this thickened middle part the division of the cartilage into two parts to form the ends of two contiguous vertebræ is subsequently effected. The curved line which this segmentation will follow is, however, already marked out, and from surface views it might be supposed that this division had actually occurred. The histological structure of the intervertebral cartilage is very distinct from that of the cartilage of the bases of the arches, the nuclei being much more closely packed. In parts, indeed, the intervertebral cartilage has almost the character of fibro-cartilage. On each side of the line of division separating two vertebræ it is invested by a superficial osseous deposit. The next oldest larva we have had was 11 centims. in length. The filamentous dorsal lobe of the caudal fin still projected far beyond the permanent caudal fin (Plate 34, fig. 16). The vertebral column was considerably less advanced in development than that dissected by Gegenbaur, though it shews a great advance on the previous stage. Its features are illustrated by two transverse sections, one through the median plane of a
- 44. vertebral region (Plate 42, fig. 78) and the other through that of an intervertebral region (Plate 42, fig. 79), and by a horizontal section (Plate 42, fig. 77). In the last stage the notochord was only constricted vertebrally. Now, however, by the great growth of intervertebral cartilage there have appeared (Plate 42, fig. 77) very well-marked intervertebral constrictions, by the completion of which the vertebræ of Lepidosteus acquire their unique character amongst Fishes. These constrictions still, however, coexist with the earlier, though at this stage relatively less conspicuous, vertebral constrictions. The gelatinous body of the notochord retains its earlier condition. The sheath has, however, undergone some changes. In the vertebral regions there is present in any section of the sheath—(1) externally, the membrana elastica externa (m.el.); then (2) the external layer of the sheath (sh.), which is, however, less thick than before, and exhibits a very faint form of radial striation; and (3) internally, a fairly thick and concentrically striated layer. The whole thickness is, on an average, 0.18 millim. In the intervertebral regions the membrana elastica externa is still present in most parts, but has become absorbed at the posterior border of each vertebra, as shewn in longitudinal section in Plate 42, fig. 77. It is considerably puckered transversely. The sheath of the notochord within the membrana elastica externa is formed of a concentrically striated layer, continuous with the innermost layer of the sheath in the vertebral regions. It is puckered longitudinally. Thus, curiously enough, the membrana elastica externa and the sheath of the notochord in the intervertebral regions are folded in different directions, the folds of the one being only visible in transverse sections (Plate 42, fig. 79), and those of the other in longitudinal sections (Plate 42, fig. 77). The osseous and cartilaginous structures investing the notochord may conveniently be dealt with in the same order as before, viz.: the
- 45. neural arches, the hæmal arches, and the intervertebral cartilages. The cartilaginous portions of the neural arches are still unossified, and form (Plate 42, fig. 78, n.a.) small wedge-shaped masses resting on the sheath of the notochord. They are invested by a thick layer of bone prolonged upwards to meet the dorsal processes (n.a ´.), which are still formed of cartilage invested by bone. It will be remembered that in the last stage there was no key-stone closing in the neural arch above. This deficiency is now however supplied, and consists of (1) two bars of cartilage repeated for each vertebra, but intervertebrally placed, which are directly differentiated from the ligamentum longitudinale superius, into which they merge above; and (2) two osseous plates placed on the outer sides of these cartilages, which are continuous with the lateral osseous bars of the neural arch. The former of these elements gives rise to the cartilaginous elements above the osseous bridge of the neural arch in the adult. The two osseous plates supporting these cartilages clearly form what we have called in our description of the adult the osseous roof of the spinal canal. A comparison of the neural arch at this stage with the arch in the adult, and in the stage last described, shews that the greater part of the neural arch of the adult is formed of membrane-bone, there being preformed in cartilage only a small basal part, a dorsal process, and paired key-stones below the ligamentum longitudinale superius. The hæmal arches (Plate 42, fig. 78) are still largely cartilaginous, and rest upon the sheath of the notochord. They are invested by a thick layer of bone. The bony layer investing the neural and hæmal arches is prolonged to form a continuous investment round the vertebral portions of the notochord (Plate 42, fig. 78). This investment is at the sides prolonged outwards into irregular processes (Plate 42, fig. 78), which form the commencement of the outer part of the thick but cellular osseous cylinder forming the middle part of the vertebral body.
- 46. The intervertebral cartilages are much larger than in the earlier stage (Plate 42, figs. 77 and 79), and it is by their growth that the intervertebral constrictions of the notochord are produced. They have ceased to be continuous with the cartilage of the arches, the intervening portion of the vertebral body between the two being only formed of bone. They are not yet divided into two masses to form the contiguous ends of adjacent vertebræ. Externally, the part of each cartilage which will form the hinder end of a vertebral body is covered by a tube of bone, having the form of a truncated funnel, shewn in longitudinal section in Plate 42, fig. 77, and in transverse section in Plate 42, fig. 79. At each end, the intervertebral cartilages are becoming penetrated and replaced by beautiful branched processes from the homogeneous bone which was first of all formed in the perichondrium (Plate 42, fig. 77). This constitutes the latest stage which we have had. Gegenbaur (No. 6) has described the vertebral column in a somewhat older larva of 18 centims. The chief points in which the vertebral column of this larva differed from ours are: (1) the disappearance of all trace of the primitive vertebral constriction of the notochord; (2) the nearly completed constriction of the notochord in the intervertebral regions; (3) the complete ossification of the vertebral portions of the bodies of the vertebræ, the terminal so-called intervertebral portions alone remaining cartilaginous; (4) the complete ossification of the basal portions of the hæmal and neural processes included within the bodies of the vertebræ, so that in the case of the neural arch all trace of the fact that the greater part was originally not formed in cartilage had become lost. The cartilage of the dorsal spinous processes was, however, still persistent. The only points which remain obscure in the later history of the vertebral column are the history of the notochord and of its sheath.
- 47. We do not know how far these are either simply absorbed or partially or wholly ossified. Götte in his memoir on the formation of the vertebral bodies of the Teleostei attempts to prove (1) that the so-called membrana elastica externa of the Teleostei is not a homogeneous elastica, but is formed of cells, and (2) that in the vertebral regions ossification first occurs in it. In Lepidosteus we have met with no indication that the membrana elastica externa is composed of cells; though it is fair to Götte to state that we have not examined such isolated portions of it as he states are necessary in order to make out its structure. But further than this we have satisfied ourselves that during the earlier stage of ossification this membrane is not ossified, and indeed in part becomes absorbed in proximity to the intervertebral cartilages; and Gegenbaur met with no ossification of this membrane in the later stage described by him. Summary of the development of the vertebral column and ribs. A mesoblastic investment is early formed round the notochord, which is produced into two dorsal and two ventral ridges, the former uniting above the neural canal. Around the cuticular sheath of the notochord an elastic membrane, the membrana elastica externa, is next developed. The neural ridges become enlarged at each inter- muscular septum, and these enlargements soon become converted into cartilage, thus forming a series of neural processes riding on the membrana elastica externa, and extending about two-thirds of the way up the sides of the neural canal. The hæmal processes arise simultaneously with, and in the same manner as, the neural. They are small in the trunk, but at the front end of the anal fin they suddenly enlarge and extend ventralwards. Each succeeding pair of hæmal arches becomes larger than the one in front, each arch finally meeting its fellow below the caudal vein, thus forming a completely closed hæmal canal. These arches are moreover
- 48. produced into long spines supporting the fin-rays of the caudal fin, which thus differs from the other unpaired fins in being supported by parts of the vertebral column, and not by separately formed skeletal elements. In the next stage which we have had the opportunity of studying (larva of 5½ centims.), a series of very well-marked vertebral constrictions are to be seen in the notochord. The sheath is now much thicker in the vertebral than in the intervertebral regions: this is due to a special differentiation of a superficial part of the sheath, which appears more granular than the remainder. This granular part of the sheath thus forms a cylinder in each vertebral region. Between it and the gelatinous tissue of the notochord there remains a thin unmodified portion of the sheath, which is continuous with the intervertebral parts of the sheath. The neural and hæmal arches are seen to be continuous with a cartilaginous tube embracing the intervertebral regions of the notochord, and continuous from one vertebra to the next. A delicate layer of bone, developed in the perichondrium, invests the cartilaginous neural arches, and this bone grows upwards so as to unite above with the osseous investment of separately developed bars of cartilage, which are directed obliquely backwards. These bars, or dorsal processes, may be reckoned as parts of the neural arches. Between the dorsal processes of the two sides is placed a median rod of cartilage, which is developed separately from the true neural arches, and which constitutes the median spinous element of the adult. Immediately below this rod is placed the ligamentum longitudinale superius. There is now a commencement of separation between the dorsal and ventral parts of the hæmal arches, not only in the tail, but also in the trunk, where they pass ventralwards on each side of the body-cavity, immediately outside the peritoneal membrane, along the lines of insertion of the intermuscular septa. These are obviously the ribs of the adult, and there is no break of continuity of structure between the hæmal processes of the tail and the ribs. In the anterior part of the trunk the ribs pass outwards along the intermuscular septa till they reach the epidermis. Thus the ribs are originally continuous
- 49. with the hæmal processes. Behind the region of the ventral caudal fin the two hæmal processes merge into one, which is not perforated by a canal. Each of the intervertebral rings of cartilage becomes eventually divided into two parts, and converted into the adjacent faces of contiguous vertebræ, the curved line where this will be effected being plainly marked out. These rings are united with the neural and hæmal arches of the vertebræ next in front and behind. As these rings are formed originally by the spreading of the cartilage from the primitive neural and hæmal processes, the intervertebral cartilages are clearly derived from the neural and hæmal arches. The intervertebral cartilages are thicker in the middle than at their two ends. In our latest stage (11 centims.), the vertebral constrictions of the notochord are rendered much less conspicuous by the growth of the intervertebral cartilages giving rise to marked intervertebral constrictions. In the intervertebral regions the membrana elastica externa has become aborted at the posterior border of each vertebra, and the remaining part is considerably puckered transversely. The inner sheath of the notochord is puckered longitudinally in the intervertebral regions. The granular external layer of the sheath in the vertebral regions is less thick than in the last stage, and exhibits faint radial striations. Two closely approximated cartilaginous elements now form a key- stone to the neural arch above: these are directly differentiated from the ligamentum longitudinale superius, into which they merge above. An osseous plate is formed on the outer side of each of these cartilages. These plates are continuous with the lateral osseous bars of the neural arches, and also give rise to the osseous roof of the spinal canal of the adult. Thus the greater part of the neural arches is formed of membrane bone. The hæmal arches are invested by a thick layer of bone, and there is also a continuous osseous investment round the vertebral
- 50. portions of the notochord. The intervertebral cartilages become penetrated by branched processes of bone. Comparison of the vertebral column of Lepidosteus with that of other forms. The peculiar form of the articulatory faces of the vertebræ of Lepidosteus caused L. Agassiz (No. 2) to compare them with the vertebræ of Reptiles, and subsequent anatomists have suggested that they more nearly resemble the vertebræ of some Urodelous Amphibia than those of any other form. If, however, Götte's account of the formation of the amphibian vertebræ is correct, there are serious objections to a comparison between the vertebræ of Lepidosteus and Amphibia on developmental grounds. The essential point of similarity supposed to exist between them consists in the fact that in both there is a great development of intervertebral cartilage which constricts the notochord intervertebrally, and forms the articular faces of contiguous vertebræ. In Lepidosteus this cartilage is, as we have seen, derived from the bases of the arches; but in Amphibia it is held by Götte to be formed by a special thickening of a cellular sheath round the notochord which is probably homologous with the cartilaginous sheath of the notochord of Elasmobranchii, and therefore with part of the notochordal sheath placed within the membrana elastica externa. If the above statements with reference to the origin of the intervertebral cartilage in the two types are true, it is clear that no homology can exist between structures so differently developed. Provisionally, therefore, we must look elsewhere than in Lepidosteus for the origin of the amphibian type of vertebræ. The researches which we have recorded demonstrate, however, in a very conclusive manner that the vertebræ of Lepidosteus have very close affinities with those of Teleostei.
- 51. In support of this statement we may point: (1) To the structure of the sheath of the notochord; (2) to the formation of the greater part of the bodies of the vertebræ from ossification in membrane around the notochord; (3) to the early biconcave form of the vertebræ, only masked at a later period by the development of intervertebral cartilages; (4) to the character of the neural arches. This latter feature will be made very clear if the reader will compare our figures of the sections of later vertebræ (Plate 42, fig. 78) with Götte's[523] figure of the section of the vertebra of a Pike (Plate 7, fig. 1). In Götte's figure there are shewn similar intercalated pieces of cartilage to those which we have found, and similar cartilaginous dorsal processes of the vertebræ. Thus we are justified in holding that whether or no the opisthocœlous form of the vertebræ of Lepidosteus is a commencement of a type of vertebræ inherited by the higher forms, yet in any case the vertebræ are essentially built on the type which has become inherited by the Teleostei from the bony Ganoids. Part III.—The ribs of Fishes. The nature and homologies of the ribs of Fishes have long been a matter of controversy; but the subject has recently been brought forward in the important memoirs of Götte[524] on the Vertebrate skeleton. The alternatives usually adopted are, roughly speaking, these:—Either the hæmal arches of the tail are homologous throughout the piscine series, while the ribs of Ganoids and Teleostei are not homologous with those of Elasmobranchii; or the ribs are homologous in all the piscine groups, and the hæmal arches in the tail are differently formed in the different types. Götte has brought forward a great body of evidence in favour of the first view; while Gegenbaur[525] may be regarded as more especially the champion of the second view. One of us held in a recent publication[526] that the question was not yet settled, though the view that the ribs are homologous
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