INTEGRATED PRINCIPLES OF ZOOLOGY 16TH EDITION HICKMAN SOLUTIONS MANUAL
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CHAPTER 7 THE REPRODUCTIVE PROCESS
CHAPTER OUTLINE
7.1 “Omne vivum ex ovo”
A. In 1651, an English physiologist, William Harvey proposed that all life developed from the egg and this development was influenced by semen.
B. Reproduction is a property of life.
7.2. Nature of the Reproductive Process
A. Mechanisms (Figure 7.1)
1. Asexual reproduction involves only one parent.
a. There are no special reproductive organs or cells involved.
b. Genetically identical offspring are produced.
2. Sexual reproduction generally involves two parents.
a. Special germ cells (gametes) unite to form a zygote.
b. By receiving genetic material from both parents, the offspring are unique.
c. Sexual reproduction recombines parental characters and makes possible evolution of more diverse forms.
B. Asexual Reproduction: Reproduction Without Gametes
1. Neither eggs nor sperm are involved.
2. Unless mutations occur, all offspring have the same genotype and are clones of the parent
3. Asexual reproduction is widespread in archaea and eubacteria, unicellular eukaryotes and many invertebrate phyla.
4. Asexual reproduction ensures rapid increase in numbers.
5 Binary fission is common among bacteria and protozoa.
a. The parent divides by mitosis into two parts; each grows into an individual similar to the parent.
b. Binary fission can be lengthwise or transverse.
c. In multiple fission or schizogony, the nucleus divides repeatedly; cytoplasmic division produces many daughter cells.
d. Sporogony is spore formation, a form of multiple fission in parasitic protozoa.
6. Budding is unequal division of an organism.
a. The bud is an outgrowth of the parent; it develops organs and then detaches.
b. Budding occurs in cnidarians and some other animal phyla.
7. Gemmulation is formation of a new individual from an aggregation of cells from the parent individual surrounded by a resistant capsule (gemmule). (Figure 12.11)
a. Freshwater sponges survive winter in the dried or frozen body of the parent.
b. In good conditions, the enclosed cells become active, emerge and grow a new sponge.
8. Fragmentation involves a multicellular animal breaking into many fragments that become a new animal. This is seen in many anemones and hydroids.
C. Sexual Reproduction: Reproduction With Gametes
1. Bisexual Reproduction
a. Also called biparental, bisexual reproduction produces offspring from union of gametes from two genetically different parents.
b. Offspring therefore have a genotype different from either parent. (Figure 7.2)
c. Generally, individuals are male or female and produce spermatozoa or ova, respectively.
1) The female produces the ovum; it is large with stored yolk and is nonmotile.
2) The spermatozoon is produced by the male; it is small, motile and much more numerous.
d. Most vertebrates and many invertebrates have separate sexes; they are dioecious.
e. Some animals have both male and female organs; they are monoecious or hermaphrodites
f. Meiosis (duplication and two divisions) produces four haploid cells.
g. In fertilization, 2 haploid cells combine to restore the diploid chromosome number in zygote.
h. A zygote divides by mitosis for all somatic (body) cells.
i. Many unicellular organisms can reproduce both sexually and asexually.
j. When sexual parents merely join together to exchange nuclear material (conjugation), distinct sexes do not occur.
k. Organs that produce germ cells are gonads; testes produce sperm and ovaries produce eggs.
l. Gonads are primary sex organs; some animals lack any other “accessory” sex organs.
m. Additional accessory sex organs include penis, vagina, oviducts and uterus.
2. Hermaphroditism (Figure 7.3)
a. Hermaphrodites have both male and female organs in the same individual.
b. Many sessile, burrowing and/or endoparasitic invertebrate animals and a few fish are hermaphroditic.
c. Most avoid self-fertilization and exchange germ cells with another member of the same species.
d. Each individual is reproductive, in contrast to dioecious species where about half is male.
e. In sequential hermaphroditism, a fish starts life as one sex and is genetically programmed to change to the other sex later.
3. Parthenogenesis
a. Parthenogenesis is the development of an embryo from an unfertilized egg.
b. The male and female nuclei may fail to unite after fertilization.
c. In ameiotic parthenogenesis, no meiosis occurs and the egg forms by mitotic division.
d. In meiotic parthenogenesis, the haploid ovum is formed by meiosis and develops without fusion with the male nuclei.
1) Sperm may be absent or they may only activate development.
2) In some species, the haploid egg returns to a diploid condition by chromosomal duplication.
e. Haplodiploidy occurs in bees, wasps and ants.
1) The queen controls whether the eggs are laid fertilized or unfertilized.
2) Fertilized eggs become female workers or queens; the unfertilized eggs become drones.
f. Some desert lizards have modified meiosis so offspring are clones of the female parent.
g. Parthenogenesis avoids the energy and dangers of bringing two sexes together; but it narrows the diversity available for adaptation to new conditions.
4. Why do so many animals reproduce sexually rather than asexually? (Figure 7.4)
a. Sexual reproduction is more common among animals.
b. The costs of sexual reproduction are greater.
1) It is more complicated, requires more time and uses more energy than asexual.
2) The cost of meiosis to the female is passage of only half of her genes to offspring.
3) Production of males reduces resources for females that could produce eggs.
c. Sexual organisms produce more novel genotypes to survive in times of environmental change.
d. Asexual organisms can have more offspring in a short time to colonize new environments.
e. In crowded habitats, selection is intense and diversity prevents extinction.
f. Sexual recombination provides a means for the spread of beneficial gene mutations without holding back a population by deleterious ones.
g On a geological time scale, asexual lineages with less variation are prone to extinction.
h Many invertebrates with both sexual and asexual modes enjoy the advantages of both.
7.3. Origin and Maturation of Germ Cells
A. Germ Cells
1. Somatic cells are non-reproductive body cells; they differentiate, function and die before or with the animal.
2. Germ cells form gametes; the germ cell line provides a continuous line between generations.
3. Somatic cells support, protect and nourish the germ cell line.
4. The germ cell lineage may be traceable; in some invertebrates, the germ cells develop from somatic cells.
B. Migration of Germ Cells (Figure 7.5)
1. Vertebrate gonads arise from a pair of genital ridges that grows into the coelom from the dorsal coelomic lining on each side of the hindgut near the anterior end of the kidney.
2. Primordial germ cells themselves arise from yolk-sac endoderm, not the developing gonad.
3. Germ plasm from the vegetal pole of the uncleaved egg mass moves to gut endoderm and migrates by ameboid movement to genital ridges.
4. Germ cells divide first by mitosis, increasing from a few dozen to several thousand.
C. Sex Determination
1. Originally gonads are sexually indifferent.
2. In mammalian males, SRY (sex determining region Y) on the Y chromosome organizes the gonad into a testis.
3. Formed as a testis, it secretes testosterone which, with dihydrotestosterone (DHT), masculinizes the fetus, causing development of a penis, scrotum and male glands.
4. In the brain, testosterone is enzymatically converted to estrogen, which determines brain organization for male-typical behavior.
5. Recent molecular evidence indicates that the X chromosome expresses ovary-determining genes, such as WNT4 and DAX1.
6. Despite levels of estrogen, the female brain does not become masculinized perhaps due to low estrogen receptors.
7. Genetics of sex determination vary: XX-XY, XX-XO, haplodiploid, temperature, etc. [see Ch. 5]. (Figure 7.6)
D. Gametogenesis
1. Gametogenesis is the series of transformations that result in gametes.
2. Testes carry out spermatogenesis; ovaries carry out oogenesis.
3. Spermatogenesis (Figures 7.7, 7.8)
a. The wall of seminiferous tubules contains germ cells five to eight cells deep.
b. Sertoli (sustentacular) cells extend from the periphery to nourish germ cells.
c. The outermost layers are spermatogonia, diploid cells that have increased by mitosis.
d. A spermatogonium increases in size to become a primary spermatocyte
e. A primary spermatocyte undergoes the first meiotic division to become two secondary spermatocytes.
f. Without resting, each secondary spermatocyte enters the second meiotic division to produce four haploid spermatids.
g. Spermatids transform into mature spermatozoa (sperm).
1) Most cytoplasm is lost.
2) The haploid nucleus condenses into a head.
3) A midpiece forms containing mitochondria.
4) The whiplike flagellar tail provides locomotion.
h. The sperm head contains an acrosome (except for some fishes and invertebrates).
1) Often the acrosome contains lysins to clear an entrance through layers surrounding the egg.
2) In mammals, one lysin is hyaluronidase; it allows sperm to penetrate follicular cells around the egg.
3) In many invertebrate sperm, an acrosome filament extends suddenly upon contact with surface of the egg.
i. Fusion of egg and sperm plasma membranes is the initial event for fertilization.
j. Size of sperm varies from 50 µm to 2 mm in length; most are very small. (Figure 7.9)
k. Sperm greatly outnumber eggs.
4 Oogenesis
a. Oogonia are early germ cells in the ovary; they are diploid and increase by mitosis.
b. They cease to grow in number and increase in size as primary oocytes (Figure 7.10)
c. Chromosomes pair in the first meiotic division, similar to spermatogenesis.
d. In this first division, the cytoplasm is divided unequally.
e. A larger daughter cell or secondary oocyte receives most of the cytoplasm; the rest goes to the first polar body
f. In the second meiotic division, the secondary oocyte forms a large ootid and a small polar body.
g. Since the first polar body also divides, this produces three polar bodies that disintegrate.
h. The ootid forms a functional ovum with all the cytoplasmic components necessary for development.
i. Unlike spermatogenesis that forms four gametes, oogenesis forms one haploid ovum.
j. Most vertebrate and some invertebrate eggs wait for fertilization to complete the last meiotic divisions.
1) Development is arrested in prophase I in the primary oocyte phase; meiosis resumes at ovulation or after fertilization.
2) Human ova begin the first meiotic division at the thirteenth week of fetal development.
3) Human ova arrest development in prophase I until puberty.
4) After puberty, some oocytes develop into secondary oocytes; meiosis II is completed only after penetration by a spermatozoon.
k. Yolk
1) Egg maturation involves deposition of yolk.
2) Yolk is stored as granules of lipid, protein or both.
3) Yolk may be synthesized internally or supplied from follicle cells.
4) Accumulation of yolk granules and nutrients cause eggs to grow massively beyond normal cell size.
5. The size of an egg violates surface-area-to-volume ratios; it therefore slows metabolism.
7.4. Reproductive Patterns
A. Live-birth Versus Egg-bearing
1. Oviparous animals lay eggs outside the body for development.
a. Fertilization may be internal (before eggs are laid) or external (after laid).
b. Some animals abandon eggs; others provide extensive care.
2. Ovoviviparous animals retain eggs in their body.
a. Essentially all nourishment is derived from the yolk.
b. This is common in some invertebrate groups and certain fishes and reptiles.
c. Fertilization is internal.
3. Viviparous animals give live birth.
a. Eggs develop in an oviduct or uterus.
b. Embryos continuously derive nourishment from the mother.
c. Fertilization is internal.
d. This occurs in mammals and some fishes, lizards and snakes; it provides more protection to offspring.
e. Some physiologists consider ovoviviparity as a special kind of vivaparity (lecithotroph vivparity).
7.5. Structure of Reproductive Systems
A. Components
1 Primary organs are the gonads that produce sperm, eggs and sex hormones.
2. Accessory organs assist gonads in formation and delivery of gametes and may support embryos.
B. Invertebrate Reproductive Systems (Figures 7.11 and 20.6)
1 Invertebrates that transfer sperm for internal fertilization require complex organs.
2. Invertebrates that release sperm into water for external fertilization may be simple.
a. Polychaete annelids have no permanent reproductive organs; gametes are cells from the body cavity.
b. Mature gametes may be released through ducts or exit through ruptures.
3. Insects have separate sexes and accomplish internal fertilization using complex systems.
a. Sperm from testes are stored in seminal vesicles before ejaculated.
b. Female insects have ovaries in a series of egg tubes.
c. Mature ova pass to a common genital chamber and short vagina.
d. Sperm inserted by male are stored in a seminal receptacle in female.
e. One mating may provide enough sperm to last the reproductive life of a female insect.
C. Vertebrate Reproductive Systems
1. Urogenital system of vertebrates shows close connections of reproductive and excretory systems.
2. The opisthonephric duct drains the kidney and carries sperm in male fishes and amphibians.
3. The mesonephric duct is composed of the vas deferens and a separate ureter develops in male reptiles, birds and mammals.
4. The cloaca is the common chamber for intestinal, reproductive and excretory canals, except in mammals.
5. The uterine duct of the oviduct has an independent duct opening into cloaca when present.
D. Male Reproductive System
1 Paired testes are sites of sperm production.
2. Testes contain numerous seminiferous tubules where sperm develop. (Figure 7.12)
3. Sperm are surrounded by Sertoli cells that nourish developing sperm.
4. Between tubules are interstitial cells (leydig cells) that produce testosterone.
5. A sac-like scrotum suspends testes outside the warm body cavity; the lower temperature of scrotum is vital to normal sperm production.
6. Sperm pass from the testes to vasa efferentia and to coiled epididymis for maturation.
7. The vas deferens carries sperm from the epididymis to the urethra, where it exits the penis.
8. The penis is a copulatory organ used to introduce spermatozoa into the female vagina.
9. Seminal vesicles, prostate gland and bulbourethral glands form seminal fluid.
a. Seminal vesicles secrete a thick fluid containing nutrients for use by sperm.
b. The prostate gland secretes a milky, slightly alkaline solution that counters acidity.
c. Bulbourethral glands release mucus secretions that provide lubrication.
E. Female Reproductive System
1. Ovaries in female vertebrates produce ova and the female sex hormones, estrogen and progesterone.
2. In jawed vertebrates, mature ova from ovaries enter funnel-like oviducts (fallopian tube or uterine tube).
3. The terminal end of uterine tube is specialized in cartilaginous fishes, reptiles and birds to produce shelled eggs; special regions produce albumin and shell.
4. The terminal portion of amniote uterine tube expands into a muscular uterus.
a. Shelled eggs may be retained here before laying.
b. Embryos may complete their development here.
c. Placental mammals use the walls of the uterus to intermingle vascular tissue as a placenta.
5. Ovaries are paired and slightly smaller than male testes. (Figure 7.13)
a. Oocytes develop within a follicle that enlarges to release a secondary oocyte.
b. Unless fertilization occurs, women release about 13 oocytes per year, 300–400 per a 30-year reproductive lifetime.
c. 300–400 primary oocytes, of ca. 400,000 in ovaries at birth, reach maturity while the rest degenerate and are absorbed.
6. Uterine tubes or oviducts are lined with cilia that propel the egg.
7. The oviducts enter the upper corners of the uterus.
8. Uterus
a. The uterus is specialized to house the embryo for nine months.
b. The uterus has thick muscular walls and is stretchable.
c. The endometrium is the specialized lining rich in blood vessels.
d. Ancestrally, the uterus was paired but is fused in eutherian mammals.
9. The vagina is a muscular tube that receives the male’s penis and serves as the birth canal.
10. The cervix is the end of the uterus that extends into the vagina.
11. The vulva is external genitalia in human females.
a. Labia majora and labia minora enclose urethral and vaginal openings.
b. The clitoris is a small erectile organ equivalent to the glans penis of male.
7.6. Endocrine Events that Orchestrate Reproduction
A. Hormonal Control of Timing of Reproductive Cycles
1. Vertebrate reproduction is seasonal or cyclic to align with food supply and survival of young.
2. Sexual cycles are controlled by hormones that respond to food intake, photoperiod, rainfall, temperature or social cues.
3. The hypothalamus region of the forebrain regulates the release of anterior pituitary gland hormones, which stimulate tissues of the gonads.
4. Estrous Cycles
a. Females are receptive to males only during brief periods of estrus or “heat.”
b. The estrous cycle ends with uterine lining reverting to original state; there is no menstruation.
5. Menstrual Cycles
a. This cycle occurs in monkeys, apes and humans.
b. Females are receptive to males throughout the cycle.
c. At the end of the menstrual cycle the endometrium (uterine lining) is discharged.
B. Gonadal Steroids and Their Control (Figure 7.14)
1. Ovaries produce estrogens and progesterone.
2. The three estrogens include estradiol, estrone and estriol.
3. Estrogen Functions
a. Estrogens develop female accessory sex structures: oviducts, uterus and vagina.
b. Estrogens stimulate female reproductive activity.
c. Secondary non-reproductive characteristics include
1) skin or feather coloration,
2) bone development,
3) body size, and
4) initial development of mammary glands in mammals.
4. Both estrogen and progesterone prepare the uterus to receive an embryo.
a. The hypothalamus produces gonadotropin releasing hormone (GnRH).
b. GnRH governs pituitary release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
c. Light, nutrition, stress, etc. can influence this complex feedback system.
5. Testosterone
a. Interstitial cells in testes manufacture testosterone.
b. Testosterone and its metabolite dihydrotestosterone (DHT) are required for growth of the penis, sperm ducts, and glands, and secondary sexual traits.
c. Secondary non-reproductive characteristics include
1) male plumage and pelage coloration,
2) bone and muscle growth,
3) antlers in deer, and
4) vocal cord growth in humans.
d. Testosterone and DHT feedback to hypothalamus and anterior pituitary to keep secretion of GnRH, FSH and LH in check.
e. Sertoli cells of testes secrete inhibin; it regulates FSH of anterior pituitary by negative feedback.
C. The Menstrual Cycle (Figure 7.15)
1. The ovary has two phases: follicular and luteal.
2. The uterus has three phases: menstrual, proliferative and secretory.
3. Menstruation, shedding of the uterine lining, signals the menstrual phase
4. The follicular phase of ovary is also occurring.
a. By day three of the menstrual cycle, blood levels of FSH and LH rise slowly, prompting some ovarian follicles to grow and secrete estrogen.
b. As estrogen increases, the uterine endometrium heals and begins to thicken.
c. Uterine glands within the endometrium enlarge in the proliferative phase of uterus
d. By day 10, most ovarian follicles degenerate (become atretic) leaving one, two or three to continue ripening.
e. Final mature follicle is the Graafian follicle; it secretes more estrogen and also inhibin.
f. At day 13 or 14, high levels of estrogen from the Graafian follicle stimulate a surge in GnRH from hypothalamus.
g. This stimulates a surge of LH and some FSH from anterior pituitary.
h. The LH surge causes the largest follicle to rupture and release an oocyte (ovulation).
5. The luteal phase of the ovary is named for the corpus luteum, which is the remainder of the ruptured follicle.
a. The corpus luteum responds to LH and secretes progesterone
b. Progesterone stimulates the uterus to undergo maturation and prepare for gestation.
c. If an embryo implants, the uterus enters the secretory phase.
d. If fertilization does not occur, the corpus luteum degenerates and hormones are no longer secreted.
e. The uterus depends on progesterone and estrogen to maintain uterine lining; declining levels start endometrium degeneration and lead to menstrual discharge.
6. Negative feedback among the hypothalamus, anterior pituitary and ovary control the cycle.
7. Recently a possible gonadotropin-inhibiting hormone has been discovered in the hypothalamus of birds and mammals.
8. Ovulation is due to high levels of estrogen causing a surge in GnRH, LH and FSH; such positive feedback is rare since it moves events away from stable set points.
D. Hormones of Human Pregnancy and Birth (Figure 7.16)
1. Fertilization normally occurs in the outer third of the uterine tube (ampulla).
2. As the zygote travels to uterus, it divides by mitosis to form a blastocyst.
3. In about six days, on contact with the uterine lining, it embeds in the endometrium (implantation).
4. The spherically-shaped trophoblast contains three layers: amnion, chorion and embryo proper.
5. The chorion is a source of human chorionic gonadotropin (hCG); it stimulates the corpus luteum to produce estrogen and progesterone.
6. The placenta is formed between the trophoblast and uterus.
a. The placenta is an endocrine gland, secreting hCG, estriol, and progesterone.
b. The placenta also transfers nutrients and wastes between mother and fetus.
c. After a month, the corpus luteum degenerates and the placenta itself holds the lining by progesterone and estrogen. (Figure 7.17)
7. Preparation of mammary glands to secrete milk requires two additional hormones.
a. Prolactin (PRL) is produced by the anterior pituitary, but is inhibited in non-pregnant women.
b. During pregnancy, elevated progesterone and estrogen depress inhibition and PRL appears in the blood.
c. PRL is also produced by the placenta during pregnancy.
d. Human placental lactogen (hPL) aids PRL in preparing the mammary glands for secretion.
e. Together with maternal growth hormone, hPL stimulates an increase in nutrients in the mother.
f. The placenta also secretes -endorphin and other endogenous opioids that regulate appetite and mood during pregnancy.
g. The placenta later synthesizes peptide hormone relaxin to allow expansion of pelvis by flexibility of pubic symphysis. (Figure 29.9)
h. Relaxin also dilates the cervix in preparation for delivery.
8. Birth or parturition begins with rhythmic contractions of uterus called labor.
a. Placental corticotropin-releasing hormone (CRH) appears to initiate the birth process.
b. Estrogen secreted before birth stimulates contractions.
c. Progesterone levels, which inhibit contraction, decline.
d. Prostaglandin hormones increase, making the uterus more irritable.
e. Uterine stretching causes neural reflexes to stimulate secretion of oxytocin from the posterior pituitary.
f. Oxytocin stimulates uterine smooth muscle contractions.
g. Childbirth (Figure 7.18)
1) First stage: the cervix enlarges and the amniotic sac will rupture.
2) Second stage: the baby is forced out of the uterus and through the vagina.
3) Third stage: the placenta or afterbirth is expelled.
h. Milk Production
1) Milk production is triggered when infant sucks on the mother’s nipple.
2) Stimulation leads to reflex release of oxytocin from the pituitary.
3) Oxytocin causes contraction of smooth muscles lining ducts of mammary glands.
4) Suckling also stimulates release of prolactin, which continues milk production.
E. Multiple Births (Figure 7.19)
1. Many mammals are multiparous, giving birth to many offspring at one time.
2. Some give birth only to one at a time; they are uniparous.
3. Exceptions occur; the armadillo gives birth to four young, all male or all female, derived from one zygote.
4. Monozygotic, or identical, twins are derived from one zygote; they have identical genomes.
5. Fraternal, dizygotic or nonidentical, twins are from two zygotes and may not resemble each other any more than other siblings.
6. Identical Twins
a. They may separate early and have separate placentas.
b. Two-thirds share a placenta and splitting occurred after formation of the inner cell mass, but most have individual amniotic sacs.
c. A few share one amniotic sac and a single placenta; separation of the zygote occurred after day 9 of pregnancy when the amnion has formed; these twins risk becoming conjoined (Siamese twinning).
Lecture Enrichment
1. Discuss the procession to internal egg fertilization as a requirement for life on land. Contrast the shelled eggs of reptiles and birds, the egg cases of insects and the internal development of the fetus in mammals. Discuss how each is adapted to prevent desiccation.
2. Have students trace the path of sperm from its development in the testes to its release during ejaculation; ask for the additions of glandular secretions along the way.
3. Research shows that when sperm enters the egg, the mitochondrial mid-piece and parts of the tail also enter. However, there are far fewer sperm mitochondria and they are worn out, and the egg tags them for cell destruction. Thus, a person inherits all of his mitochondria from his/her biological mother.
4. Have students trace the path of the egg as it develops in the ovary and is released. They should describe what happens to the egg if it is fertilized or if it is not fertilized.
5. The description of the hormonal feedback of the menstrual cycle is sufficient to allow students to think through the mechanism of the birth control pill.
Commentary/Lesson Plan
Background: Due to American society’s preoccupation with this topic, this is generally the highest interest topic in biology. Student experiences in this subject may be varied; students from rural areas may still have experiences with animal birth, breeding, litters, etc. However, the social context may prevent using some student testimonials and experiences.
Misconceptions: The term “germ” has far more recognition in meaning as a pathogen or microbe than as “germinal lineage.” It is also not easy for students to comprehend the germ line as an immortal cell lineage where we are individual temporary support systems for this cell lineage a particularly biological viewpoint. “Womb” is a nonfunctional term since it has historically been used for ovaries as well as uterus. “Hermaphrodites” and “bisexual” have social meanings different from their biological usage here. Ambiguous human sexual development is rarely truly hermaphroditic (see John Money and Anke Erhardt’s Man and Woman, Boy and Girl), but hermaphroditism has genuine advantages for organisms isolated in soil or hosts. Woody Allen’s famous quote that bisexuality doubles your chances of a date on Friday evening clearly explains the advantages of hermaphroditism and also reveals the different social usage of “bisexual” from the scientific meaning for this term. Basic bisexual
reproduction refers to the separate male and female organism system we use, but it will be difficult for some students to associate this with human heterosexuality.
Schedule: Time spent on this chapter may vary greatly depending on how much an instructor wishes to elaborate on the examples and wide diversity of animal reproductive systems.
HOUR 1 7.1 “Omen vivum ex ovo”
7.2. Nature of the Reproductive Process
A. Mechanisms
B. Asexual Reproduction: Reproduction Without Gametes
C. Sexual Reproduction: Reproduction With Gametes
7.3. Origin and Maturation of Germ Cells
A. Germ Cells
B. Migration of Germ Cells
C. Sex Determination
D. Gametogenesis
HOUR 2 7.4. Reproductive Patterns
A. Live-birth Versus Egg-bearing
7.5. Structure of Reproductive Systems
A. Components
B. Invertebrate Reproductive Systems
C. Vertebrate Reproductive Systems
D. Male Reproductive System
E. Female Reproductive System
ADVANCED CLASS QUESTIONS:
HOUR 3 7.6. Endocrine Events that Orchestrate Reproduction
A. Hormonal Control of Timing of Reproductive Cycles
B. Gonadal Steroids and Their Control
C. Menstrual Cycle
D. Hormones of Human Pregnancy and Birth
E. Multiple Births
1. About one-half of a herd of cattle is born male, but a farmer does not want all these troublesome tough-meat bulls. Therefore, most are castrated before puberty. What physiological and body changes would you expect, and would the steer resemble the cow or bull and why?
2. Twinning rates vary around the earth, with twins occurring in one out of 80 births in the U.S., with higher frequency in the tropics where life-span is much shorter, and with less frequency in northern regions and Asia where life-span has historically been longer. Age of first menstruation is also lower in the tropics, higher in Norway, etc. What are the physiological basis and the evolutionary implications of this?
3. If some reptiles (e.g., garter snakes) hold eggs internally until they hatch, why has this strategy not evolved in all snakes?
4. Why would you predict hormonal rather than nervous control of the reproductive cycle?
5. Why would a woman who was having difficulty conceiving due to too low hormone levels be more likely to have multiple births when treated?
6. How might some mechanisms triggering contractions cause too early a delivery when a mother is carrying multiples?
7. Why would evolution not select for triggering milk production in humans based simply on a clock-like mechanism that “went off” at nine months?