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35 Lecture Ppt

  1. 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 35 Reproduction and Development
  2. 2. Reproduction in Animals Is Varied 35-
  3. 3. 35.1 Both asexual and sexual reproduction occur among animals <ul><li>Asexual Reproduction </li></ul><ul><ul><li>Several phyla of invertebrates can reproduce asexually </li></ul></ul><ul><ul><ul><li>Sponges, cnidarians, flatworms, and echinoderms </li></ul></ul></ul><ul><ul><li>New individual arises as an outgrowth (bud) of the parent </li></ul></ul><ul><ul><li>Parthenogenesis is a modification of sexual reproduction in which unfertilized egg develops into a complete individual </li></ul></ul><ul><li>Sexual Reproduction </li></ul><ul><ul><li>Egg of one parent is fertilized by the sperm of another </li></ul></ul><ul><ul><ul><li>Most are di oecious, which means having separate sexes </li></ul></ul></ul><ul><ul><li>Usually produce gametes in specialized organs called gonads </li></ul></ul><ul><ul><ul><li>Testes produce sperm, and ovaries produce eggs </li></ul></ul></ul><ul><ul><li>External fertilization - egg and sperm are released into the water </li></ul></ul><ul><ul><li>Copulation is sexual union to facilitate the reception of sperm by a female </li></ul></ul><ul><ul><ul><li>Males typically have a penis for depositing sperm into the vagina of females </li></ul></ul></ul>35-
  4. 4. Figure 35.1A Hydras can reproduce asexually 35-
  5. 5. Figure 35.1B Orange-fin anemonefish, Amphiprion chrysopterus , tend an egg mass 35-
  6. 6. Figure 35.1C Azure damselflies mating on land 35-
  7. 7. 35.2 Development in water and on land occurs among animals <ul><li>Ovoviviparous animals retain their eggs in various ways and release young able to fend for themselves </li></ul><ul><ul><li>Reptiles lay a leathery-shelled egg containing extraembryonic membranes to serve the needs of the embryo and prevent drying out </li></ul></ul><ul><ul><li>One membrane surrounds an abundant supply of yolk , a nutrient-rich material </li></ul></ul><ul><li>Marsupials and placental mammals do not lay eggs </li></ul><ul><ul><li>In marsupials, yolk sac membrane functions briefly to supply the unborn with nutrients from the mother </li></ul></ul><ul><ul><ul><li>Immature young finish their development within a pouch where they are nourished on milk </li></ul></ul></ul><ul><ul><li>Placental mammals are viviparous because they do not lay eggs and development occurs inside the female’s body until offspring can live independently </li></ul></ul><ul><ul><li>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 </li></ul></ul>35-
  8. 8. Figure 35.2 Parenting in birds 35-
  9. 9. Humans Are Adapted to Reproducing on Land 35-
  10. 10. 35.3 Testes are male gonads <ul><li>In the human male, paired testes produce sperm and are suspended within scrotum </li></ul><ul><ul><li>Sperm produced by testes mature within epididymis , a tubule outside each testis </li></ul></ul><ul><ul><li>Once matured, they are propelled into the vas deferens by muscular contractions </li></ul></ul><ul><li>Penis - cylindrical organ that hangs in front of scrotum </li></ul><ul><ul><li>An enlarged tip normally covered by a layer of skin called the foreskin </li></ul></ul><ul><ul><li>During sexual arousal, an increase in arterial blood flow to the penis creates an erection </li></ul></ul><ul><li>Semen (seminal fluid) a thick, whitish fluid that contains sperm and secretions from three types of glands </li></ul><ul><ul><li>Seminal vesicles lie at the base of the bladder produces a viscous fluid containing nutrients for possible use by the sperm </li></ul></ul><ul><ul><li>Prostate gland , which secretes a milky alkaline fluid believed to activate or increase the motility of the sperm </li></ul></ul><ul><ul><li>Bulbourethral glands secrete mucus that has a lubricating effect </li></ul></ul>35-
  11. 11. Figure 35.3 Male reproductive system 35-
  12. 12. Male Orgasm <ul><li>If sexual arousal reaches its peak, ejaculation follows an erection </li></ul><ul><ul><li>First phase, emission , the spinal cord sends nerve impulses via appropriate nerve fibers to epididymis and vas deferens </li></ul></ul><ul><ul><ul><li>Subsequent motility causes sperm to enter the ejaculatory duct, whereupon the seminal vesicles, prostate gland, and bulbourethral glands release their secretions </li></ul></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><ul><li>Contractions are a part of male orgasm, physiological and psychological sensations that occur at climax of sexual stimulation </li></ul></ul></ul><ul><li>Following ejaculation, a male typically experiences a time, called the refractory period, during which stimulation does not bring about an erection </li></ul>35-
  13. 13. 35.4 Production of sperm and male sex hormones occurs in the testes <ul><li>Seminiferous tubules inside testes are packed with cells undergoing spermatogenesis , production of sperm </li></ul><ul><ul><li>Sertoli cells , which support, nourish, and regulate the production of sperm </li></ul></ul><ul><li>Sperm has three distinct parts </li></ul><ul><ul><li>Tail - flagellum that allows a sperm to swim toward the egg </li></ul></ul><ul><ul><li>Middle piece - contains the energy-producing mitochondria </li></ul></ul><ul><ul><li>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 </li></ul></ul>35-
  14. 14. Figure 35.4 Seminiferous tubules, where sperm are produced via the process of spermatogenesis 35-
  15. 15. Control of Testes Function <ul><li>Hypothalamus has ultimate control of the testes’ sexual function because it secretes a hormone called gonadotropin-releasing hormone, or GnRH </li></ul><ul><ul><li>Follicle-stimulating hormone ( FSH ) promotes spermatogenesis in the seminiferous tubules </li></ul></ul><ul><ul><li>Luteinizing hormone ( LH ) controls the production of testosterone by the interstitial cells </li></ul></ul><ul><li>Testosterone - main sex hormone in males, is essential for the normal development and functioning of the sexual organs and the maturation of sperm </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>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 </li></ul></ul>35-
  16. 16. 35.5 Ovaries are female gonads <ul><li>In the human female, ovaries , which lie in shallow depressions, one on each side of the upper pelvic cavity </li></ul><ul><ul><li>Produce one oocyte each month </li></ul></ul><ul><li>Oviducts , also called uterine or fallopian tubes, extend from the ovaries to the uterus </li></ul><ul><ul><li>Oviducts are not attached to the ovaries, but have fingerlike projections called fimbriae (sing., fimbria) that lie over the ovaries </li></ul></ul><ul><li>Fertilization normally takes place in an oviduct, and the developing embryo is propelled slowly by ciliary movement and tubular muscle contraction to the uterus </li></ul><ul><ul><li>Uterus is a thick-walled muscular organ about the size and shape of an inverted pear </li></ul></ul><ul><ul><li>Narrow end of the uterus is called the cervix </li></ul></ul><ul><ul><li>Embryo completes its development after embedding itself in the uterine lining, called the endometrium </li></ul></ul><ul><li>Vagina is a tube at a 45-degree angle to the small of the back </li></ul><ul><ul><li>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 </li></ul></ul>35-
  17. 17. Figure 35.5A Female reproductive system 35-
  18. 18. External Genital Organs and Orgasm <ul><li>In the female, the external genital organs are known collectively as the vulva </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>Beneath them are small glands (Bartholin glands) on either side of the vagina that keep the vulva moist and lubricated during intercourse </li></ul></ul><ul><li>At the juncture of the labia minora is the clitoris , which is homologous to the penis in males </li></ul><ul><ul><li>Has a shaft of erectile tissue and is capped by a pea-shaped glans </li></ul></ul><ul><ul><li>Clitoris has twice as many nerve endings as the penis </li></ul></ul><ul><ul><li>Orgasm in the female is a release of neuromuscular tension in the muscles of the genital area, vagina, and uterus </li></ul></ul>35-
  19. 19. Figure 35.5B Vulva 35-
  20. 20. 35.6 Production of oocytes and female sex hormones occurs in the ovaries <ul><li>Oocyte Production </li></ul><ul><ul><li>An oocyte is produced as a follicle changes from a primary to a secondary to a vesicular (Graafian) follicle </li></ul></ul><ul><ul><li>Epithelial cells of a primary follicle surround a primary oocyte </li></ul></ul><ul><ul><li>Pools of follicular fluid surround oocyte in a secondary follicle </li></ul></ul><ul><ul><li>In a vesicular follicle, a fluid-filled cavity increases to the point that the follicle wall balloons out on the surface of the ovary </li></ul></ul><ul><ul><li>Vesicular follicle bursts, releasing an oocyte surrounded by a clear membrane </li></ul></ul><ul><ul><ul><li>This process is referred to as ovulation </li></ul></ul></ul><ul><ul><li>Once a vesicular follicle has lost its oocyte, it develops into a corpus luteum </li></ul></ul><ul><li>Oocyte maturation requires oogenesis , a form of meiosis </li></ul><ul><ul><li>If fertilization occurs, a sperm enters the secondary oocyte, and then the oocyte completes meiosis </li></ul></ul><ul><ul><ul><li>When the sperm nucleus unites with the egg nucleus, a zygote with 46 chromosomes is produced </li></ul></ul></ul><ul><ul><ul><li>If zygote formation and pregnancy do not occur, the corpus luteum begins to degenerate after about ten days </li></ul></ul></ul>35-
  21. 21. Figure 35.6A Ovarian follicles, where eggs are produced 35-
  22. 22. Figure 35.6B Oogenesis 35-
  23. 23. Hormone Production <ul><li>Ovaries also produce the female sex hormones, estrogens (collectively called estrogen) and progesterone during the ovarian cycle </li></ul><ul><ul><li>Estrogen, in particular, is essential for the normal development and functioning of the female reproductive organs </li></ul></ul><ul><ul><li>Estrogen is also largely responsible for the secondary sex characteristics in females, including body hair and fat distribution </li></ul></ul><ul><ul><li>Pelvic girdle enlarges so females have wider hips than males, and the thighs converge at a greater angle toward the knees </li></ul></ul><ul><ul><li>Both estrogen and progesterone are required for breast development as well </li></ul></ul>35-
  24. 24. 35.7 The ovarian cycle drives the uterine cycle <ul><li>Ovarian Cycle - Ovaries go through the same series of events each month </li></ul><ul><ul><li>Follicular phase - anterior pituitary produces the follicle-stimulating hormone (FSH) </li></ul></ul><ul><ul><ul><li>FSH promotes the development of a follicle that secretes estrogen and some progesterone </li></ul></ul></ul><ul><ul><li>Hypothalamus secretes a large amount of gonadotrophic-releasing hormone (GnRH) </li></ul></ul><ul><ul><ul><li>Leads to a surge in LH production by anterior pituitary and ovulation at about the fourteenth day </li></ul></ul></ul><ul><ul><li>During the second half, or luteal phase , of the ovarian cycle, luteinizing hormone (LH) promotes the development of corpus luteum, which primarily secretes progesterone </li></ul></ul>35-
  25. 25. Uterine Cycle <ul><li>Hormones produced during the ovarian cycle affect the endometrium, causing the cyclical series of events known as the uterine cycle </li></ul><ul><ul><li>Days 1–5: Level of female sex hormones in the body is low, causing the endometrium to disintegrate and its blood vessels to rupture </li></ul></ul><ul><ul><ul><li>Blood flow out of the vagina during menstruation </li></ul></ul></ul><ul><ul><li>Days 6–13: Increased production of estrogen by an ovarian follicle causes the endometrium to thicken and become vascular and glandular </li></ul></ul><ul><ul><ul><li>Called the proliferative phase </li></ul></ul></ul><ul><ul><li>Day 14: Ovulation usually occurs </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><ul><li>Secretory phase of the uterine cycle and the endometrium is now prepared to receive the developing embryo </li></ul></ul></ul>35-
  26. 26. Figure 35.7 Female hormone levels during the ovarian and uterine cycles 35-
  27. 27. APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 35.8 Sexual activity can transmit disease <ul><li>Sexually transmitted disease (STD): passed between persons during sex </li></ul><ul><li>Herpes Simplex Virus (HSV): HSV-1 usually affects mouth, causing “cold-sores,” HSV-2 usually infects genitals and anus </li></ul><ul><ul><li>Clusters of blisters arise, rupture, and form sores </li></ul></ul><ul><li>Human papillomavirus (HPV): Causes genital warts and cervical cancer </li></ul><ul><ul><li>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 </li></ul></ul><ul><li>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 </li></ul><ul><ul><li>Four out of five infected women do not experience any symptoms, although some have painful urination, vaginal bleeding, and/or abdominal pain </li></ul></ul><ul><li>Syphilis: caused by the spirochete bacterium Treponema pallidum </li></ul><ul><ul><li>Open sore may appear at or near the site where the bacteria entered the body </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><li>Gonorrhea: caused by the bacterium Neisseria gonorrhoeae </li></ul><ul><ul><li>In men, the bacteria may infect the urethra, causing painful urination and pus-filled urine </li></ul></ul><ul><ul><li>In women, cervical infection may go unnoticed for several months. Meanwhile, the bacteria spread throughout the reproductive organs, often leading to PID </li></ul></ul><ul><li>Trichomoniasis: Usually the parasite infects the urethra in men and the vagina in women </li></ul>35-
  28. 28. 35-
  29. 29. APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 35.9 Numerous birth control methods are available <ul><li>Figure 35.9 Contraceptive devices </li></ul>35-
  30. 30. 35-
  31. 31. APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 35.10 Reproductive technologies are available to help the infertile <ul><li>Infertility - inability of a couple to achieve pregnancy after one year of regular, unprotected intercourse </li></ul><ul><ul><li>Cause of infertility can be attributed to the male (40%), the female (40%), or both (20%) </li></ul></ul><ul><li>Artificial Insemination by Donor (AID) </li></ul><ul><ul><li>Sperm are placed in the vagina by a physician </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><li>In Vitro Fertilization (IVF) </li></ul><ul><ul><li>Union of a sperm and an egg to form a zygote, occurs in laboratory glassware </li></ul></ul><ul><li>Gamete Intrafallopian Transfer (GIFT) </li></ul><ul><ul><li>Same as in vitro fertilization, except the eggs and the sperm are placed in the oviducts immediately after they have been brought together </li></ul></ul><ul><li>Surrogate Mothers </li></ul><ul><ul><li>In some instances, women are contracted and paid to have babies </li></ul></ul><ul><li>Intracytoplasmic Sperm Injection (ICSI) </li></ul><ul><ul><li>A single sperm is injected into an egg </li></ul></ul>35-
  32. 32. Figure 35.10 Fertilization can occur in the laboratory 35-
  33. 33. Vertebrates Have Similar Early Developmental Stages and Processes 35-
  34. 34. 35.11 Cellular stages of development precede tissue stages <ul><li>Cellular stages of development </li></ul><ul><ul><li>Cleavage resulting in a multicellular embryo </li></ul></ul><ul><ul><ul><li>Cell division without growth </li></ul></ul></ul><ul><ul><li>Formation of the blastula, a hollow ball of cells with a fluid-filled cavity called a blastocoel </li></ul></ul><ul><li>Human blastula resembles chick embryo because both birds and mammals are related to reptiles </li></ul>35-
  35. 35. 35-
  36. 36. Figure 35.11 Lancelet early development 35-
  37. 37. 35.12 Tissue stages of development precede organ stages <ul><li>Tissue stages of development are early gastrula and late gastrula </li></ul><ul><ul><li>Cells continue to migrate during other stages of development </li></ul></ul><ul><li>An early gastrula has two layers of cells </li></ul><ul><ul><li>Outer layer is called the ectoderm </li></ul></ul><ul><ul><li>Inner layer is called the endoderm </li></ul></ul><ul><ul><ul><li>Endoderm borders a primitive gut </li></ul></ul></ul><ul><ul><ul><ul><li>Pore, or hole, created by invagination is the blastopore , and in a lancelet, the blastopore eventually becomes the anus </li></ul></ul></ul></ul><ul><li>Gastrulation is not complete until three layers of cells that will develop into adult organs are produced </li></ul><ul><ul><li>In addition to ectoderm and endoderm, late gastrula has a middle layer of cells, the mesoderm </li></ul></ul>35-
  38. 38. Figure 35.12 Comparative development of mesoderm 35-
  39. 39. 35-
  40. 40. 35.13 Organ stages of development occur after tissue stages <ul><li>Newly formed mesoderm cells lie along main longitudinal axis of animal and coalesce to form a presumptive notochord </li></ul><ul><ul><li>Cells mark the location of the future dorsal supporting rod called the notochord </li></ul></ul><ul><li>Formation of the neural tube </li></ul><ul><ul><li>Thickening of cells, neural plate , appears along the dorsal surface of the embryo </li></ul></ul><ul><ul><li>Neural folds develop on either side of neural groove </li></ul></ul><ul><ul><li>Coelom appears and neural tube is complete </li></ul></ul><ul><li>At this point, the embryo is called a neurula </li></ul><ul><ul><li>Anterior end of the neural tube develops into the brain , and the rest becomes the spinal cord </li></ul></ul>35-
  41. 41. Figure 35.13A Development of neural tube an coelom in a frog embryo 35-
  42. 42. Figure 35.13B Vertebrate embryo, cross section 35-
  43. 43. 35.14 Cellular differentiation begins with cytoplasmic segregation <ul><li>Development requires growth, cellular differentiation, and morphogenesis </li></ul><ul><ul><li>Morphogenesis produces the shape and form of the body </li></ul></ul><ul><ul><ul><li>Cell movement - one of the earliest indications of morphogenesis </li></ul></ul></ul><ul><ul><ul><li>Morphogenesis includes pattern formation , way that tissues and organs are arranged in body </li></ul></ul></ul>35-
  44. 44. Cytoplasmic Segregation <ul><li>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 </li></ul><ul><li>If gray crescent is divided equally by first cleavage, each separated daughter cell develops into a complete embryo </li></ul><ul><li>If egg divides so that only one daughter cell receives the gray crescent, only that cell becomes a complete embryo </li></ul>35-
  45. 45. 35-
  46. 46. Figure 35.14 Cytoplasmic influence on development 35-
  47. 47. 35.15 Morphogenesis involves induction also <ul><li>Induction - ability of one embryonic tissue to influence development of another tissue </li></ul><ul><ul><li>Gray crescent of a frog’s egg marks the dorsal side of embryo, where mesoderm becomes notochord and ectoderm becomes nervous system </li></ul></ul><ul><li>Homeotic Genes </li></ul><ul><ul><li>Most likely, signaling molecules during induction turn on homeotic genes that code for homeotic proteins, which may be involved in morphogen gradients </li></ul></ul>35-
  48. 48. Figure 35.15 Control of nervous system development 35-
  49. 49. Human Development Is Divided into Embryonic Development and Fetal Development 35-
  50. 50. 35.16 Extraembryonic membranes are critical to human development <ul><li>In humans, length of time from conception (fertilization followed by implantation ) to birth is about nine months (266 days) </li></ul><ul><li>In humans, pregnancy, or gestation, is the time during which the mother carries the developing embryo </li></ul><ul><li>Human development divided into two parts </li></ul><ul><ul><li>Embryonic development (months 1 and 2) </li></ul></ul><ul><ul><ul><li>Major organs are formed </li></ul></ul></ul><ul><ul><li>Fetal development (months 3–9) </li></ul></ul><ul><ul><ul><li>Structures are refined </li></ul></ul></ul>35-
  51. 51. Extraembryonic Membranes <ul><li>In chicks chorion lies next to the shell and carries on gas exchange </li></ul><ul><ul><li>The amnion contains the protective amniotic fluid, which bathes the developing embryo </li></ul></ul><ul><ul><li>The allantois collects nitrogenous wastes, and the yolk sac surrounds the remaining yolk, which provides nourishment </li></ul></ul><ul><li>Humans (and other mammals) also have extraembryonic membranes </li></ul><ul><ul><li>Chorion develops into fetal half of the placenta, where nutrient for waste exchange occurs between the fetus and mother </li></ul></ul><ul><ul><li>Yolk sac, which lacks yolk, is the first site of blood cell formation </li></ul></ul><ul><ul><li>Blood vessels of the allantois become umbilical blood vessels </li></ul></ul><ul><ul><li>Amnion contains fluid to cushion and protect the embryo, which develops into a fetus </li></ul></ul>35-
  52. 52. Figure 35.16 Extraembryonic membranes 35-
  53. 53. 35.17 Embryonic development involves tissue and organ formation <ul><li>Cleavage begins 30 hours after fertilization and continues as embryo passes through oviduct to uterus </li></ul><ul><li>Seventh day morula is transformed into blastocyst </li></ul><ul><ul><li>Fluid-filled cavity called the amniotic cavity </li></ul></ul><ul><ul><li>Single layer of outer cells called the trophoblast and an inner cell mass </li></ul></ul><ul><li>Embryo begins the process of implanting in wall of the uterus </li></ul><ul><ul><li>Trophoblast begins to secrete human chorionic gonadotropin (HCG) that maintains corpus luteum past the time it normally disintegrates </li></ul></ul><ul><ul><li>Inner cell mass detaches itself from the trophoblast, and two more extraembryonic membranes form </li></ul></ul><ul><ul><ul><li>Gastrulation occurs during the second week and inner cell mass now has flattened into embryonic disk , composed of ectoderm above and endoderm below </li></ul></ul></ul>35-
  54. 54. Figure 35.17A Overview of human embryonic development 35-
  55. 55. During Third Week the Embryonic Disk Becomes the Embryo <ul><li>Nervous and circulatory systems are the first visually evident organ systems </li></ul><ul><li>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 </li></ul><ul><ul><li>Body stalk and the yolk sac fuse to become the umbilical cord , which connects the developing embryo to the placenta </li></ul></ul><ul><li>Fifth week little flippers called limb buds appear </li></ul><ul><li>Sixth to eighth weeks of development, the embryo becomes easily recognizable as human </li></ul>35-
  56. 56. Figure 35.17B Human embryo at fifth week 35-
  57. 57. 35.18 Fetal development involves refinement and weight gain <ul><li>Fetal development occurs during the third through the ninth months of development </li></ul><ul><ul><li>Placenta is now fully developed </li></ul></ul><ul><ul><ul><li>Has a fetal side contributed by the chorion, the outermost extraembryonic membrane, and a maternal side consisting of uterine tissues </li></ul></ul></ul><ul><ul><ul><li>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 </li></ul></ul></ul><ul><li>At the beginning of the third month </li></ul><ul><ul><li>Cartilage begins to be replaced by bone </li></ul></ul><ul><ul><ul><li>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 </li></ul></ul></ul><ul><ul><li>It is possible to distinguish males from females </li></ul></ul><ul><li>During the fourth month </li></ul><ul><ul><li>Fetal heartbeat is loud enough to be heard when a physician applies a stethoscope to the mother’s abdomen </li></ul></ul><ul><ul><li>Fetal movement felt by women who have previously been pregnant </li></ul></ul>35-
  58. 58. Figure 35.18A 9-week fetus 35-
  59. 59. Figure 35.18B 12- to 16-week fetus 35-
  60. 60. Fifth Through Ninth Month of Gestation <ul><li>During the fifth through seventh months </li></ul><ul><ul><li>Wrinkled, translucent, pink-colored skin covered by down called lanugo </li></ul></ul><ul><ul><ul><li>Coated with a white, greasy, cheeselike substance called vernix caseosa </li></ul></ul></ul><ul><li>At the end of seventh month, fetus’s length has increased to about 15”, and weight is about 3 lb </li></ul><ul><ul><li>If born now, the baby will survive </li></ul></ul><ul><li>Toward end of development fetus usually assumes fetal position </li></ul><ul><ul><li>Bead bent down and in contact with flexed knees </li></ul></ul><ul><ul><li>Eventually, the body rotates so that head points toward cervix </li></ul></ul><ul><ul><li>If the fetus does not turn, a breech birth (rump first) is likely </li></ul></ul><ul><ul><ul><li>Cesarean section (incision through the abdominal and uterine walls) may be prescribed for delivery of the fetus </li></ul></ul></ul><ul><li>At the end of nine months </li></ul><ul><ul><li>Fetus is about 20½ in. long and weighs about 7½ lb </li></ul></ul>35-
  61. 61. Figure 35.18C 20- to 28-week fetus 35-
  62. 62. Figure 35.18D 24-week fetus 35-
  63. 63. Figure 35.18E Newborn (40 weeks) 35-
  64. 64. 35.19 Pregnancy ends with the birth of the newborn <ul><li>Uterus undergoes contractions throughout pregnancy </li></ul><ul><ul><li>At first, light, lasting 20–30 seconds and occurring every 15–20 minutes </li></ul></ul><ul><ul><ul><li>Near the end of pregnancy, the contractions may become stronger and more frequent so that a woman thinks she is in labor </li></ul></ul></ul><ul><ul><ul><ul><li>“ False-labor” contractions are called Braxton Hicks contractions </li></ul></ul></ul></ul><ul><li>Stage 1 </li></ul><ul><ul><li>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 </li></ul></ul><ul><li>Stage 2 </li></ul><ul><ul><li>Uterine contractions occur every 1–2 minutes and last one minute </li></ul></ul><ul><ul><ul><li>Accompanied by a desire to push, or bear down </li></ul></ul></ul><ul><ul><li>Vaginal orifice may not expand enough to allow passage of the head, an episiotomy is often performed </li></ul></ul><ul><ul><ul><li>This incision, which enlarges the opening, is sewn together later </li></ul></ul></ul><ul><li>Stage 3 </li></ul><ul><ul><li>Placenta, or afterbirth , is delivered </li></ul></ul><ul><ul><li>About 15 minutes after delivery of the baby, uterine muscular contractions shrink the uterus and dislodge the placenta </li></ul></ul>35-
  65. 65. Connecting the Concepts: Chapter 35 <ul><li>Reproduction in animals is varied, depending on their complexity and the environment in which they live </li></ul><ul><ul><li>Sexual reproduction always produces a zygote that undergoes development to become a functioning offspring </li></ul></ul><ul><ul><li>We have observed that animals go through the same early embryonic stages of morula, blastula, gastrula, and so forth </li></ul></ul><ul><li>The set sequence of these stages is due to the expression of genes </li></ul><ul><ul><li>Hormones are signals that affect cellular metabolism </li></ul></ul><ul><ul><li>Steroid hormones in animals turn on the expression of genes </li></ul></ul><ul><ul><ul><li>Signal transduction pathways also occur during development </li></ul></ul></ul><ul><li>A set sequence of signaling molecules is produced as development occurs </li></ul><ul><ul><li>Each new signal in the sequence turns on a specific gene or, more likely, a sequence of genes </li></ul></ul><ul><li>Developmental biology is now making a significant contribution to the field of evolution </li></ul><ul><ul><li>Homeotic (pattern formation) genes have been discovered in many different types of organisms </li></ul></ul><ul><ul><li>Suggests that homeotic genes arose early in the history of life, and mutations in these genes </li></ul></ul><ul><ul><li>could possibly account for macroevolution—the appearance of new species, or even higher taxa </li></ul></ul>35-