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Chapter 35
Reproduction
and
Development

Reproduction in
Animals Is Varied
35-2

35.1 Both asexual and sexual
reproduction occur among animals
 Asexual Reproduction
 Several phyla of invertebrates can reproduce asexually
 Sponges, cnidarians, flatworms, and echinoderms
 New individual arises as an outgrowth (bud) of the parent
 Parthenogenesis is a modification of sexual reproduction in
which unfertilized egg develops into a complete individual
 Sexual Reproduction
 Egg of one parent is fertilized by the sperm of another
 Most are dioecious, which means having separate sexes
 Usually produce gametes in specialized organs called gonads
 Testes produce sperm, and ovaries produce eggs
 External fertilization - egg and sperm are released into the water
 Copulation is sexual union to facilitate the reception of sperm
by a female
 Males typically have a penis for depositing sperm into the vagina of
females
35-3

Figure 35.1A Hydras can reproduce asexually
35-4

Figure 35.1B Orange-fin anemonefish, Amphiprion chrysopterus,
tend an egg mass
35-5

Figure 35.1C Azure damselflies mating on land
35-6

35.2 Development in water and
on land occurs among animals
 Ovoviviparous animals retain their eggs in various ways and
release young able to fend for themselves
 Reptiles lay a leathery-shelled egg containing extraembryonic
membranes to serve the needs of the embryo and prevent drying
out
 One membrane surrounds an abundant supply of yolk, a nutrient-
rich material
 Marsupials and placental mammals do not lay eggs
 In marsupials, yolk sac membrane functions briefly to supply the
unborn with nutrients from the mother
 Immature young finish their development within a pouch where they
are nourished on milk
 Placental mammals are viviparous because they do not lay eggs
and development occurs inside the female’s body until offspring
can live independently
 Placenta is a complex structure that allows the developing young
to internally exchange materials with the mother until the offspring
can function on their own 35-7

Figure 35.2 Parenting in birds
35-8

Humans Are Adapted to
Reproducing on Land
35-9

35.3 Testes are male gonads
 In the human male, paired testes produce sperm and are
suspended within scrotum
 Sperm produced by testes mature within epididymis, a tubule
outside each testis
 Once matured, they are propelled into the vas deferens by
muscular contractions
 Penis - cylindrical organ that hangs in front of scrotum
 An enlarged tip normally covered by a layer of skin called the
foreskin
 During sexual arousal, an increase in arterial blood flow to the
penis creates an erection
 Semen (seminal fluid) a thick, whitish fluid that contains sperm
and secretions from three types of glands
 Seminal vesicles lie at the base of the bladder produces a viscous
fluid containing nutrients for possible use by the sperm
 Prostate gland, which secretes a milky alkaline fluid believed to
activate or increase the motility of the sperm
 Bulbourethral glands secrete mucus that has a lubricating effect
35-10

Figure 35.3 Male reproductive system
35-11

Male Orgasm
 If sexual arousal reaches its peak, ejaculation
follows an erection
 First phase, emission, the spinal cord sends nerve
impulses via appropriate nerve fibers to epididymis and
vas deferens
 Subsequent motility causes sperm to enter the ejaculatory
duct, whereupon the seminal vesicles, prostate gland, and
bulbourethral glands release their secretions
 Second phase, expulsion, rhythmic contractions of
muscles at the base of the penis and within the urethral
wall expel semen in spurts from opening of the urethra
 Contractions are a part of male orgasm, physiological and
psychological sensations that occur at climax of sexual
stimulation
 Following ejaculation, a male typically experiences a
time, called the refractory period, during which
stimulation does not bring about an erection 35-12

35.4 Production of sperm and male
sex hormones occurs in the testes
 Seminiferous tubules inside testes are packed
with cells undergoing spermatogenesis, production
of sperm
 Sertoli cells, which support, nourish, and regulate the
production of sperm
 Sperm has three distinct parts
 Tail - flagellum that allows a sperm to swim toward the egg
 Middle piece - contains the energy-producing mitochondria
 Head - contains a nucleus and is capped by a membrane-
bounded acrosome that contains enzymes that assist in
allowing a sperm to enter an egg during fertilization
35-13

Figure 35.4 Seminiferous tubules, where sperm are produced via
the process of spermatogenesis
35-14

Control of Testes Function
 Hypothalamus has ultimate control of the testes’ sexual
function because it secretes a hormone called
gonadotropin-releasing hormone, or GnRH
 Follicle-stimulating hormone (FSH) promotes
spermatogenesis in the seminiferous tubules
 Luteinizing hormone (LH) controls the production of
testosterone by the interstitial cells
 Testosterone - main sex hormone in males, is essential
for the normal development and functioning of the sexual
organs and the maturation of sperm
 Brings about and maintains the male secondary sex
characteristics that develop at the time of puberty the time of
life when sexual maturity is attained
 Testosterone causes males to develop noticeable hair on the
face, chest, and the deeper voice of males compared to females
is due to males having a larger larynx with longer vocal cords
35-15

35.5 Ovaries are female gonads
 In the human female, ovaries, which lie in shallow depressions, one
on each side of the upper pelvic cavity
 Produce one oocyte each month
 Oviducts, also called uterine or fallopian tubes, extend from the
ovaries to the uterus
 Oviducts are not attached to the ovaries, but have fingerlike projections
called fimbriae (sing., fimbria) that lie over the ovaries
 Fertilization normally takes place in an oviduct, and the developing
embryo is propelled slowly by ciliary movement and tubular muscle
contraction to the uterus
 Uterus is a thick-walled muscular organ about the size and shape of an
inverted pear
 Narrow end of the uterus is called the cervix
 Embryo completes its development after embedding itself in the uterine
lining, called the endometrium
 Vagina is a tube at a 45-degree angle to the small of the back
 Mucosal lining of the vagina lies in folds, and the vagina can distend,
which is especially important when the vagina serves as the birth canal,
and it can also facilitate sexual intercourse, when the vagina receives
the penis 35-16

Figure 35.5A Female reproductive system
35-17

External Genital
Organs and Orgasm
 In the female, the external genital organs are known
collectively as the vulva
 Mons pubis and two sets of skin folds called the labia minora
(sing., labium minus) and labia majora (sing., labium majus) are
on either side of the urethral and vaginal openings
 Beneath them are small glands (Bartholin glands) on either side
of the vagina that keep the vulva moist and lubricated during
intercourse
 At the juncture of the labia minora is the clitoris, which
is homologous to the penis in males
 Has a shaft of erectile tissue and is capped by a pea-shaped
glans
 Clitoris has twice as many nerve endings as the penis
 Orgasm in the female is a release of neuromuscular tension in
the muscles of the genital area, vagina, and uterus
35-18

35.6 Production of oocytes and female
sex hormones occurs in the ovaries
 Oocyte Production
 An oocyte is produced as a follicle changes from a primary to a
secondary to a vesicular (Graafian) follicle
 Epithelial cells of a primary follicle surround a primary oocyte
 Pools of follicular fluid surround oocyte in a secondary follicle
 In a vesicular follicle, a fluid-filled cavity increases to the point that
the follicle wall balloons out on the surface of the ovary
 Vesicular follicle bursts, releasing an oocyte surrounded by a clear
membrane
 This process is referred to as ovulation
 Once a vesicular follicle has lost its oocyte, it develops into a
corpus luteum
 Oocyte maturation requires oogenesis, a form of meiosis
 If fertilization occurs, a sperm enters the secondary oocyte, and
then the oocyte completes meiosis
 When the sperm nucleus unites with the egg nucleus, a zygote with 46
chromosomes is produced
 If zygote formation and pregnancy do not occur, the corpus luteum
begins to degenerate after about ten days 35-20

Figure 35.6A Ovarian follicles, where eggs are produced
35-21

Hormone Production
 Ovaries also produce the female sex hormones,
estrogens (collectively called estrogen) and
progesterone during the ovarian cycle
 Estrogen, in particular, is essential for the normal
development and functioning of the female
reproductive organs
 Estrogen is also largely responsible for the secondary
sex characteristics in females, including body hair and
fat distribution
 Pelvic girdle enlarges so females have wider hips
than males, and the thighs converge at a greater
angle toward the knees
 Both estrogen and progesterone are required for
breast development as well 35-23

35.7 The ovarian cycle drives
the uterine cycle
 Ovarian Cycle - Ovaries go through the same
series of events each month
 Follicular phase - anterior pituitary produces the
follicle-stimulating hormone (FSH)
 FSH promotes the development of a follicle that secretes
estrogen and some progesterone
 Hypothalamus secretes a large amount of
gonadotrophic-releasing hormone (GnRH)
 Leads to a surge in LH production by anterior pituitary and
ovulation at about the fourteenth day
 During the second half, or luteal phase, of the
ovarian cycle, luteinizing hormone (LH) promotes the
development of corpus luteum, which primarily
secretes progesterone
35-24

Uterine Cycle
 Hormones produced during the ovarian cycle affect the
endometrium, causing the cyclical series of events
known as the uterine cycle
 Days 1–5: Level of female sex hormones in the body is low,
causing the endometrium to disintegrate and its blood vessels to
rupture
 Blood flow out of the vagina during menstruation
 Days 6–13: Increased production of estrogen by an ovarian
follicle causes the endometrium to thicken and become vascular
and glandular
 Called the proliferative phase
 Day 14: Ovulation usually occurs
 Days 15–28: Increased production of progesterone by the
corpus luteum causes endometrium to double in thickness and
the uterine glands to mature, producing a thick mucoid secretion
 Secretory phase of the uterine cycle and the endometrium is now
prepared to receive the developing embryo
35-25

Figure 35.7 Female hormone levels during the ovarian and uterine
cycles
35-26

APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES
35.8 Sexual activity can
transmit disease
 Sexually transmitted disease (STD): passed between persons during sex
 Herpes Simplex Virus (HSV): HSV-1 usually affects mouth, causing “cold-
sores,” HSV-2 usually infects genitals and anus
 Clusters of blisters arise, rupture, and form sores
 Human papillomavirus (HPV): Causes genital warts and cervical cancer
 Women aged 18–65 are encouraged to have an annual Pap smear, in which cells are
removed from the cervix and examined to see if any are transforming into cancer cells
 Chlamydial infection: Most common STD in the U.S., frequently occurs in the
urethra or cervix, but may also exist in the rectum or throat
 Four out of five infected women do not experience any symptoms, although some have
painful urination, vaginal bleeding, and/or abdominal pain
 Syphilis: caused by the spirochete bacterium Treponema pallidum
 Open sore may appear at or near the site where the bacteria entered the body
 Rash may develop over the entire body or just on the hands and feet and if not treated
can affect the heart, brain and bones
 Gonorrhea: caused by the bacterium Neisseria gonorrhoeae
 In men, the bacteria may infect the urethra, causing painful urination and pus-filled urine
 In women, cervical infection may go unnoticed for several months. Meanwhile, the
bacteria spread throughout the reproductive organs, often leading to PID
 Trichomoniasis: Usually the parasite infects the urethra in men and the
vagina in women 35-27

35.9 Numerous birth control
methods are available
Figure 35.9 Contraceptive devices 35-29

35.10 Reproductive technologies
are available to help the infertile
 Infertility - inability of a couple to achieve pregnancy after one year
of regular, unprotected intercourse
 Cause of infertility can be attributed to the male (40%), the female
(40%), or both (20%)
 Artificial Insemination by Donor (AID)
 Sperm are placed in the vagina by a physician
 Intrauterine insemination (IUI) - fertility drugs are given to stimulate the
ovaries, and then the donor’s sperm is placed in the uterus, rather than
in the vagina
 In Vitro Fertilization (IVF)
 Union of a sperm and an egg to form a zygote, occurs in laboratory
glassware
 Gamete Intrafallopian Transfer (GIFT)
 Same as in vitro fertilization, except the eggs and the sperm are placed
in the oviducts immediately after they have been brought together
 Surrogate Mothers
 In some instances, women are contracted and paid to have babies
 Intracytoplasmic Sperm Injection (ICSI)
 A single sperm is injected into an egg
35-31

Figure 35.10 Fertilization can occur in the laboratory
35-32

Vertebrates Have Similar Early
Developmental Stages
and Processes
35-33

35.11 Cellular stages of
development precede
tissue stages
 Cellular stages of development
 Cleavage resulting in a multicellular embryo
 Cell division without growth
 Formation of the blastula, a hollow ball of cells with a
fluid-filled cavity called a blastocoel
 Human blastula resembles chick embryo
because both birds and mammals are related to
reptiles
35-34

Figure 35.11
Lancelet early
development
35-36

35.12 Tissue stages of
development precede organ stages
 Tissue stages of development are early gastrula and late
gastrula
 Cells continue to migrate during other stages of development
 An early gastrula has two layers of cells
 Outer layer is called the ectoderm
 Inner layer is called the endoderm
 Endoderm borders a primitive gut
 Pore, or hole, created by invagination is the blastopore, and in a
lancelet, the blastopore eventually becomes the anus
 Gastrulation is not complete until three layers of cells
that will develop into adult organs are produced
 In addition to ectoderm and endoderm, late gastrula has a
middle layer of cells, the mesoderm
35-37

Figure 35.12 Comparative development of mesoderm
35-38

35.13 Organ stages of development
occur after tissue stages
 Newly formed mesoderm cells lie along main longitudinal
axis of animal and coalesce to form a presumptive
notochord
 Cells mark the location of the future dorsal supporting rod called
the notochord
 Formation of the neural tube
 Thickening of cells, neural plate, appears along the dorsal
surface of the embryo
 Neural folds develop on either side of neural groove
 Coelom appears and neural tube is complete
 At this point, the embryo is called a neurula
 Anterior end of the neural tube develops into the brain, and the
rest becomes the spinal cord
35-40

Figure 35.13A Development of neural tube an coelom in a frog
embryo
35-41

Figure 35.13B
Vertebrate
embryo, cross
section
35-42

35.14 Cellular differentiation begins
with cytoplasmic segregation
 Development requires growth, cellular
differentiation, and morphogenesis
 Morphogenesis produces the shape and form of the
body
 Cell movement - one of the earliest indications of
morphogenesis
 Morphogenesis includes pattern formation, way that tissues
and organs are arranged in body
35-43

Cytoplasmic Segregation
 Egg is polar and has both an anterior/posterior axis and
a dorsal/ventral axis, which can be correlated with the
gray crescent, a gray area that appears after the sperm
fertilizes egg
 If gray crescent is divided equally by first cleavage, each
separated daughter cell develops into a complete
embryo
 If egg divides so that only one daughter cell receives the
gray crescent, only that cell becomes a complete embryo
35-44

Figure 35.14 Cytoplasmic influence on development
35-46

35.15 Morphogenesis
involves induction also
 Induction - ability of one embryonic tissue to
influence development of another tissue
 Gray crescent of a frog’s egg marks the dorsal side of
embryo, where mesoderm becomes notochord and
ectoderm becomes nervous system
 Homeotic Genes
 Most likely, signaling molecules during induction turn
on homeotic genes that code for homeotic proteins,
which may be involved in morphogen gradients
35-47

Figure 35.15 Control of nervous system development
35-48

Human Development Is Divided
into Embryonic Development
and Fetal Development
35-49

35.16 Extraembryonic membranes
are critical to human development
 In humans, length of time from conception
(fertilization followed by implantation) to birth is
about nine months (266 days)
 In humans, pregnancy, or gestation, is the time
during which the mother carries the developing
embryo
 Human development divided into two parts
 Embryonic development (months 1 and 2)
 Major organs are formed
 Fetal development (months 3–9)
 Structures are refined
35-50

Extraembryonic Membranes
 In chicks chorion lies next to the shell and carries on
gas exchange
 The amnion contains the protective amniotic fluid, which bathes
the developing embryo
 The allantois collects nitrogenous wastes, and the yolk sac
surrounds the remaining yolk, which provides nourishment
 Humans (and other mammals) also have extraembryonic
membranes
 Chorion develops into fetal half of the placenta, where nutrient
for waste exchange occurs between the fetus and mother
 Yolk sac, which lacks yolk, is the first site of blood cell formation
 Blood vessels of the allantois become umbilical blood vessels
 Amnion contains fluid to cushion and protect the embryo, which
develops into a fetus
35-51

Figure 35.16
Extraembryonic
membranes
35-52

35.17 Embryonic development
involves tissue and organ formation
 Cleavage begins 30 hours after fertilization and
continues as embryo passes through oviduct to uterus
 Seventh day morula is transformed into blastocyst
 Fluid-filled cavity called the amniotic cavity
 Single layer of outer cells called the trophoblast and an inner
cell mass
 Embryo begins the process of implanting in wall of the
uterus
 Trophoblast begins to secrete human chorionic gonadotropin
(HCG) that maintains corpus luteum past the time it normally
disintegrates
 Inner cell mass detaches itself from the trophoblast, and two
more extraembryonic membranes form
 Gastrulation occurs during the second week and inner cell mass
now has flattened into embryonic disk, composed of ectoderm
above and endoderm below 35-53

Figure 35.17A
Overview of
human embryonic
development
35-54

During Third Week the Embryonic
Disk Becomes the Embryo
 Nervous and circulatory systems are the first
visually evident organ systems
 At four weeks, a bridge of mesoderm called the
body stalk connects the caudal (tail) end of the
embryo with the chorion, which has treelike
projections called chorionic villi
 Body stalk and the yolk sac fuse to become the
umbilical cord, which connects the developing
embryo to the placenta
 Fifth week little flippers called limb buds appear
 Sixth to eighth weeks of development, the
embryo becomes easily recognizable as human
35-55

Figure 35.17B Human embryo at fifth week
35-56

35.18 Fetal development involves
refinement and weight gain
 Fetal development occurs during the third through the ninth
months of development
 Placenta is now fully developed
 Has a fetal side contributed by the chorion, the outermost extraembryonic
membrane, and a maternal side consisting of uterine tissues
 Carbon dioxide and other wastes move from fetal side to maternal side of
the placenta, and nutrients and oxygen move from maternal side to fetal side
 At the beginning of the third month
 Cartilage begins to be replaced by bone
 Skull has six large membranous areas called fontanels, which permit a
certain amount of flexibility as the head passes through the birth canal and
allow rapid growth of the brain during infancy
 It is possible to distinguish males from females
 During the fourth month
 Fetal heartbeat is loud enough to be heard when a physician
applies a stethoscope to the mother’s abdomen
 Fetal movement felt by women who have previously been
pregnant 35-57

Figure 35.18A 9-week fetus
35-58

Figure 35.18B 12- to 16-week fetus
35-59

Fifth Through Ninth
Month of Gestation
 During the fifth through seventh months
 Wrinkled, translucent, pink-colored skin covered by down called
lanugo
 Coated with a white, greasy, cheeselike substance called vernix
caseosa
 At the end of seventh month, fetus’s length has
increased to about 15”, and weight is about 3 lb
 If born now, the baby will survive
 Toward end of development fetus usually assumes fetal
position
 Bead bent down and in contact with flexed knees
 Eventually, the body rotates so that head points toward cervix
 If the fetus does not turn, a breech birth (rump first) is likely
 Cesarean section (incision through the abdominal and uterine
walls) may be prescribed for delivery of the fetus
 At the end of nine months
 Fetus is about 20½ in. long and weighs about 7½ lb 35-60

Figure 35.18C 20- to 28-week fetus
35-61

Figure 35.18D 24-week fetus
35-62

Figure 35.18E Newborn (40 weeks)
35-63

35.19 Pregnancy ends with the
birth of the newborn
 Uterus undergoes contractions throughout pregnancy
 At first, light, lasting 20–30 seconds and occurring every 15–20 minutes
 Near the end of pregnancy, the contractions may become stronger and more
frequent so that a woman thinks she is in labor
 “False-labor” contractions are called Braxton Hicks contractions
 Stage 1
 Uterine contractions occur in such a way that the cervical canal slowly
disappears as lower part of the uterus is pulled upward toward the baby’s
head
 Stage 2
 Uterine contractions occur every 1–2 minutes and last one minute
 Accompanied by a desire to push, or bear down
 Vaginal orifice may not expand enough to allow passage of the head, an
episiotomy is often performed
 This incision, which enlarges the opening, is sewn together later
 Stage 3
 Placenta, or afterbirth, is delivered
 About 15 minutes after delivery of the baby, uterine muscular contractions
shrink the uterus and dislodge the placenta 35-64

Connecting the Concepts:
Chapter 35
 Reproduction in animals is varied, depending on their complexity and the
environment in which they live
 Sexual reproduction always produces a zygote that undergoes development to
become a functioning offspring
 We have observed that animals go through the same early embryonic stages of
morula, blastula, gastrula, and so forth
 The set sequence of these stages is due to the expression of genes
 Hormones are signals that affect cellular metabolism
 Steroid hormones in animals turn on the expression of genes
 Signal transduction pathways also occur during development
 A set sequence of signaling molecules is produced as development occurs
 Each new signal in the sequence turns on a specific gene or, more likely, a
sequence of genes
 Developmental biology is now making a significant contribution to the field of
evolution
 Homeotic (pattern formation) genes have been discovered in many different
types of organisms
 Suggests that homeotic genes arose early in the history of life, and mutations in
these genes
 could possibly account for macroevolution—the appearance of new species, or
even higher taxa 35-65

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