7-1 CHAPTER 7 The Reproductive Process

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-3 Nature of the Reproductive Process Reproduction is one of the most important properties of life Reproduction is one of the most important properties of life Two modes of reproduction are recognized Two modes of reproduction are recognized Asexual Asexual Sexual Sexual

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-5 Asexual Reproduction Asexual Reproduction Involves only one parent Involves only one parent No special reproductive organs or cells No special reproductive organs or cells Genetically identical offspring are produced Genetically identical offspring are produced Production of offspring is simple, direct, and rapid Production of offspring is simple, direct, and rapid Widespread in bacteria, unicellular eukaryotes and many invertebrate phyla Widespread in bacteria, unicellular eukaryotes and many invertebrate phyla Ensures rapid increase in numbers Ensures rapid increase in numbers Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-6 Asexual Reproductive Methods Asexual Reproductive Methods Binary Fission Binary Fission Common among bacteria and protozoa Common among bacteria and protozoa The parent divides by mitosis into two parts The parent divides by mitosis into two parts Each grows into an individual similar to the parent Each grows into an individual similar to the parent Multiple Fission Multiple Fission Nucleus divides repeatedly Nucleus divides repeatedly Cytoplasmic division produces many daughter cells Cytoplasmic division produces many daughter cells Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-7 Budding Budding Unequal division of an organism Unequal division of an organism Bud is an outgrowth of the parent Bud is an outgrowth of the parent Develops organs and then detaches Develops organs and then detaches Occurs in cnidarians (hydra, coral) and several other animal phyla Occurs in cnidarians (hydra, coral) and several other animal phyla Fragmentation Fragmentation Multicellular animal breaking into many fragments that become a new animal Multicellular animal breaking into many fragments that become a new animal Many anemones Many anemones Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-8 Sexual Reproduction Sexual Reproduction Generally involves two parents Generally involves two parents Special germ cells (gametes) unite to form a zygote Special germ cells (gametes) unite to form a zygote S exual reproduction recombines parental characters S exual reproduction recombines parental characters A richer, more diversified population results A richer, more diversified population results Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-9 Sexual Reproductive Methods Sexual Reproductive Methods Bisexual Reproduction Bisexual Reproduction Most common form Most common form Produces offspring from union of gametes from two genetically different parents Produces offspring from union of gametes from two genetically different parents Generally, individuals are male or female Generally, individuals are male or female Organisms are dioecious Organisms are dioecious Sexes are separate Sexes are separate Gonads (Found in most vertebrates and invertebrates) Gonads (Found in most vertebrates and invertebrates) Organs that produce gametes (testes, ovaries) Organs that produce gametes (testes, ovaries) Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The female produces the ovum The female produces the ovum Large with stored yolk and nonmotile Large with stored yolk and nonmotile Spermatozoa (sperm) are produced by the male Spermatozoa (sperm) are produced by the male Small, motile and much more numerous Small, motile and much more numerous Meiosis - used to make gametes Meiosis - used to make gametes Produces four haploid cells Produces four haploid cells Fertilization Fertilization Two haploid cells combine Two haploid cells combine Restores the diploid chromosome number in the zygote Restores the diploid chromosome number in the zygote Zygote divides by mitosis Zygote divides by mitosis Nature of the Reproductive Process

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hermaphroditism Hermaphroditism Both male and female organs in the same individual (monoeicious, hermaphrodites) Both male and female organs in the same individual (monoeicious, hermaphrodites) Many sessile, burrowing and/or endoparasitic invertebrates and some fish Many sessile, burrowing and/or endoparasitic invertebrates and some fish Most avoid self-fertilization Most avoid self-fertilization Exchange gametes with member of same species Exchange gametes with member of same species Each individual produces eggs, increases # Each individual produces eggs, increases # Hermaphroditic species could potentially produce twice as many offspring as dioecious species Hermaphroditic species could potentially produce twice as many offspring as dioecious species Sequential Hermaphroditism Sequential Hermaphroditism A genetically programmed sex change occurs with an individual organism A genetically programmed sex change occurs with an individual organism Nature of the Reproductive Process

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Parthenogenesis Parthenogenesis Development of an embryo from an unfertilized egg Development of an embryo from an unfertilized egg Male and female nuclei fail to unite after fertilization Male and female nuclei fail to unite after fertilization Egg begins development without sperm Egg begins development without sperm Narrows the diversity available for adaptation to new conditions Narrows the diversity available for adaptation to new conditions Examples: fleas, bees, aphids, some fish and lizards Examples: fleas, bees, aphids, some fish and lizards Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Haplodiploidy Haplodiploidy Occurs in bees, wasps and ants Occurs in bees, wasps and ants Queen controls whether the eggs are fertilized or unfertilized Queen controls whether the eggs are fertilized or unfertilized Fertilized eggs Fertilized eggs Become female workers or queens Become female workers or queens Unfertilized eggs become drones Unfertilized eggs become drones Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Why do so many animals reproduce sexually rather than asexually? The costs of sexual reproduction are greater than asexual methods The costs of sexual reproduction are greater than asexual methods More complicated More complicated Requires more time Requires more time Uses more energy Uses more energy The cost of meiosis to the female is passage of only half of her genes to offspring The cost of meiosis to the female is passage of only half of her genes to offspring Production of males reduces resources for females that could produce eggs Production of males reduces resources for females that could produce eggs Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display However: Sexual organisms Sexual organisms Produce more novel genotypes to survive in times of environmental change Produce more novel genotypes to survive in times of environmental change In crowded habitats In crowded habitats Selection is intense and diversity prevents extinction Selection is intense and diversity prevents extinction On a geological time scale On a geological time scale Sexual lineages with less variation are prone to extinction Sexual lineages with less variation are prone to extinction Many invertebrates with both sexual and asexual modes enjoy the advantages of both Many invertebrates with both sexual and asexual modes enjoy the advantages of both Nature of the Reproductive Process

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Unisex - ALL female lizardsBisexual - Both Male and Female Present

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cell Types Somatic Cells Somatic Cells Non-reproductive body cells Non-reproductive body cells Differentiate, function and die before or with the animal Differentiate, function and die before or with the animal Germ Cells Germ Cells Form gametes Form gametes Provides a continuity of life between generations Provides a continuity of life between generations

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sex Determination Sex Determination At first, gonads are sexually identical At first, gonads are sexually identical In human males In human males SRY (sex determining region Y) on the Y chromosome organizes the gonad into a testis SRY (sex determining region Y) on the Y chromosome organizes the gonad into a testis Once formed, the testis Once formed, the testis Secretes testosterone which, masculinizes the fetus Secretes testosterone which, masculinizes the fetus Development of a penis, scrotum and male ducts, and glands Development of a penis, scrotum and male ducts, and glands Sex Determination

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display In the brain In the brain Testosterone is converted to estrogen Testosterone is converted to estrogen Estrogen levels determines brain organization for male-typical behavior Estrogen levels determines brain organization for male-typical behavior Sex Determination

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Absence of testosterone in a genetic female embryo Absence of testosterone in a genetic female embryo Promotes development of female sexual organs Promotes development of female sexual organs Vagina, clitoris and uterus Vagina, clitoris and uterus Despite high levels of estrogen Despite high levels of estrogen Female brain does not become masculinized Female brain does not become masculinized Perhaps due to low levels of estrogen receptors Perhaps due to low levels of estrogen receptors Genetics of sex determination vary: Genetics of sex determination vary: XX-XY XX-XY Haplodiploid (males are formed from unfertilized eggs) Haplodiploid (males are formed from unfertilized eggs) Temperature Temperature Sex Determination

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gametogenesis Gametogenesis Gametes formation Gametes formation Spermatogenesis Spermatogenesis Sperm formation (Testes) Sperm formation (Testes) Occurs in seminiferous tubules of testes Occurs in seminiferous tubules of testes Oogenesis (Ovaries) Oogenesis (Ovaries) Germ Cells

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Spermatogenesis Spermatogenesis Spermatogonia (Diploid) Spermatogonia (Diploid) Enlarge and become primary spermatocytes (sperm) Enlarge and become primary spermatocytes (sperm) Primary Spermatocytes (Diploid) Primary Spermatocytes (Diploid) Divide by meiosis Divide by meiosis Produce 2 secondary spermatocytes Produce 2 secondary spermatocytes Secondary Spermatocytes (Haploid) Secondary Spermatocytes (Haploid) Divide by meiosis Divide by meiosis Each produce 2 spermatids Each produce 2 spermatids Spermatids (Haploid) Spermatids (Haploid) The four spermatids are transformed into 4 sperm The four spermatids are transformed into 4 sperm Sperm (Haploid) Sperm (Haploid) Size varies from 50  m to 2 mm Size varies from 50  m to 2 mm Most very small Most very small Germ Cells

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Parts of Sperm Parts of Sperm Haploid nucleus condenses into a head Haploid nucleus condenses into a head A midpiece forms containing mitochondria A midpiece forms containing mitochondria The whiplike flagellar tail provides locomotion The whiplike flagellar tail provides locomotion Sperm head contains an acrosome Sperm head contains an acrosome Often contains enzymes to aid in penetration of egg layers Often contains enzymes to aid in penetration of egg layers Germ Cells

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Oogenesis Oogenesis Oogonia (Diploid) Oogonia (Diploid) Initially increase in number by mitosis then stop dividing Initially increase in number by mitosis then stop dividing Increase in size to form primary oocytes Increase in size to form primary oocytes Primary Oocytes (Diploid) Primary Oocytes (Diploid) Divide by meiosis producing 1 large secondary ooctye and a small polar body Divide by meiosis producing 1 large secondary ooctye and a small polar body Polar bodies are nonfunctional cells Polar bodies are nonfunctional cells Secondary Ooctye (Haploid) Secondary Ooctye (Haploid) Divides by meiosis producing 1 large ootid and a second polar body Divides by meiosis producing 1 large ootid and a second polar body Ootid (Haploid) Ootid (Haploid) Develops into a functional ovum Develops into a functional ovum Each primary oocyte only forms 1 ovum Each primary oocyte only forms 1 ovum Germ Cells

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Reproductive Patterns Reproductive Patterns Oviparous (“egg-birth”) Animals Oviparous (“egg-birth”) Animals Lay eggs outside the body Lay eggs outside the body Fertilization may be internal (before eggs are laid) or external (after laid) Fertilization may be internal (before eggs are laid) or external (after laid) Some animals abandon eggs; others provide extensive care Some animals abandon eggs; others provide extensive care Examples: reptiles, birds, amphibians, fish Examples: reptiles, birds, amphibians, fish

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ovoviviparous (“egg-live-birth”) Animals Ovoviviparous (“egg-live-birth”) Animals Retain eggs in their body Retain eggs in their body Essentially all nourishment is derived from the yolk not the mother. Essentially all nourishment is derived from the yolk not the mother. Fertilization is internal Fertilization is internal Common in some invertebrate groups and aquatic animals, certain fishes (sharks) and reptiles Common in some invertebrate groups and aquatic animals, certain fishes (sharks) and reptiles Reproductive Patterns Reproductive Patterns

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Viviparous (“live-birth”) Animals Viviparous (“live-birth”) Animals Give birth to young in a more advanced stage of development Give birth to young in a more advanced stage of development Eggs develop in oviduct or uterus Eggs develop in oviduct or uterus Embryos continuously derive nourishment from the mother Embryos continuously derive nourishment from the mother Fertilization is internal Fertilization is internal Occurs in mammals and some fishes Occurs in mammals and some fishes Provides more protection to offspring Provides more protection to offspring Reproductive Patterns Reproductive Patterns

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Invertebrate Reproductive Systems Invertebrate Reproductive Systems Invertebrates that transfer sperm for internal fertilization require complex organs Invertebrates that transfer sperm for internal fertilization require complex organs Insects (Cricket) - have an ovipositor Insects (Cricket) - have an ovipositor Plan of Reproductive Systems Plan of Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Crickets’ Sex Organs Sperm is stored in a sac (spermatophore) and deposited into the genital bursa of the female. The female then controls the release of a few sperm to fertilize her eggs at the moment they are laid, using the ovipositor.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Vertebrate Reproductive Systems Vertebrate Reproductive Systems Reproductive and excretory systems are called the urogenital system Reproductive and excretory systems are called the urogenital system Close anatomical connection Close anatomical connection In male fishes and amphibians In male fishes and amphibians Wolffian duct drains the kidney and transports sperm Wolffian duct drains the kidney and transports sperm Plan of Reproductive Systems Plan of Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display In male reptiles, birds, and mammals In male reptiles, birds, and mammals Kidney develops its own independent duct to transport wastes Kidney develops its own independent duct to transport wastes Ureter Ureter The Wolffian duct becomes the vas deferens The Wolffian duct becomes the vas deferens In all vertebrates except most mammals In all vertebrates except most mammals Ducts open into a cloaca Ducts open into a cloaca In females with cloacas, the oviduct also opens into cloaca In females with cloacas, the oviduct also opens into cloaca Plan of Reproductive Systems Plan of Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Male Reproductive System Male Reproductive System Testes Testes Sperm develop within the seminiferous tubules Sperm develop within the seminiferous tubules Sertoli cells nourish the developing sperm Sertoli cells nourish the developing sperm Interstitial cells (leydig cells) Interstitial cells (leydig cells) Secrete testosterone Secrete testosterone Testes located in scrotum outside the body cavity Testes located in scrotum outside the body cavity Lower temperature of scrotum vital to normal sperm production Lower temperature of scrotum vital to normal sperm production Vertebrate Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Epididymis Epididymis Sperm pass from the testes to coiled epididymis for maturation Sperm pass from the testes to coiled epididymis for maturation Defective sperm removed Defective sperm removed Vas deferens Vas deferens Carries sperm from the epididymis to the urethra Carries sperm from the epididymis to the urethra Urethra Urethra Passes through the penis Passes through the penis Conducts urine and sperm but not simultaneously Conducts urine and sperm but not simultaneously Penis Penis Copulatory organ Copulatory organ Introduce spermatozoa into the female vagina Introduce spermatozoa into the female vagina Vertebrate Reproductive Systems Vertebrate Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Accessory Sex Glands Accessory Sex Glands Add secretions to male duct system to form seminal fluid Add secretions to male duct system to form seminal fluid Seminal vesicles Seminal vesicles Secrete a thick fluid containing nutrients for sperm Secrete a thick fluid containing nutrients for sperm Prostate gland Prostate gland Secretes a milky, slightly basic solution that counters acidity of vagina Secretes a milky, slightly basic solution that counters acidity of vagina Bulbourethral glands Bulbourethral glands Secretions provide lubrication and remove traces of urine from urethra Secretions provide lubrication and remove traces of urine from urethra Vertebrate Reproductive Systems

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Female Reproductive System Female Reproductive System Ovaries of female vertebrates Ovaries of female vertebrates Produce ova Produce ova Secrete female sex hormones, estrogen, and progesterone Secrete female sex hormones, estrogen, and progesterone Jawed vertebrates Jawed vertebrates Mature ova from ovaries enter funnel-like oviduct (aka - Fallopian tube) Mature ova from ovaries enter funnel-like oviduct (aka - Fallopian tube) Cartilaginous fishes, reptiles and birds Cartilaginous fishes, reptiles and birds Terminal end of oviducts specialized to produce shelled eggs Terminal end of oviducts specialized to produce shelled eggs Vertebrate Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Reptiles, birds, and mammals Reptiles, birds, and mammals Terminal portion of oviduct expands into a muscular uterus. Terminal portion of oviduct expands into a muscular uterus. Shelled eggs may be retained here before laying Shelled eggs may be retained here before laying Embryos may complete their development here Embryos may complete their development here Placental mammals Placental mammals Walls of uterus establish close vascular association with embryonic membranes through a placenta Walls of uterus establish close vascular association with embryonic membranes through a placenta Vertebrate Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ovaries Ovaries Female gonad Female gonad Paired organs and slightly smaller than male testes Paired organs and slightly smaller than male testes Oocytes develop within a follicle Oocytes develop within a follicle Mature follicles rupture through wall of ovary Mature follicles rupture through wall of ovary Ovum released into the pelvic cavity Ovum released into the pelvic cavity ~400,000 primary oocytes in ovaries at birth ~400,000 primary oocytes in ovaries at birth Only 300 to 400 reach maturity after puberty Only 300 to 400 reach maturity after puberty Vertebrate Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Oviducts (Fallopian Tube) Oviducts (Fallopian Tube) Lined with cilia that draw the ovum into the oviduct and on to the uterus Lined with cilia that draw the ovum into the oviduct and on to the uterus Fertilization occurs here Fertilization occurs here Uterus Uterus Site of implantation Site of implantation Endometrium Endometrium Specialized highly vascular lining of the uterus Specialized highly vascular lining of the uterus Myometrium Myometrium Thick smooth muscle layer of the uterus Thick smooth muscle layer of the uterus Vertebrate Reproductive Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Vagina Vagina Muscular tube that receives the penis and serves as birth canal Muscular tube that receives the penis and serves as birth canal Cervix Cervix Region of the uterus that extends into the vagina Region of the uterus that extends into the vagina Vulva Vulva External female genitalia External female genitalia Labia majora and labia minora enclose urethral and vaginal openings Labia majora and labia minora enclose urethral and vaginal openings Clitoris is a small erectile organ Clitoris is a small erectile organ Homologous to the penis of male Homologous to the penis of male Vertebrate Reproductive Systems Vertebrate Reproductive Systems

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Endocrine Events that Orchestrate Reproduction Hormonal Control of Timing of Reproductive Cycles Hormonal Control of Timing of Reproductive Cycles Vertebrate reproduction Vertebrate reproduction Seasonal or cyclic Seasonal or cyclic Offspring arrive when food is available and other environmental conditions are optimal for survival Offspring arrive when food is available and other environmental conditions are optimal for survival Sexual cycles Sexual cycles Controlled by hormones that respond to food intake, photoperiod, rainfall, temperature or social cues Controlled by hormones that respond to food intake, photoperiod, rainfall, temperature or social cues

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hypothalamus Hypothalamus Region of the forebrain Region of the forebrain Regulates release of anterior pituitary gland hormones, which act on tissues of the gonads Regulates release of anterior pituitary gland hormones, which act on tissues of the gonads Endocrine Events that Orchestrate Reproduction

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Preparation of Mammary Glands for Milk Secretion Preparation of Mammary Glands for Milk Secretion Prolactin (PRL) Prolactin (PRL) Hormone produced by the anterior pituitary Hormone produced by the anterior pituitary Secretion inhibited in non-pregnant women Secretion inhibited in non-pregnant women During pregnancy, elevated progesterone and estrogen depress inhibitory signal and PRL appears in the blood During pregnancy, elevated progesterone and estrogen depress inhibitory signal and PRL appears in the blood PRL also secreted by placenta PRL also secreted by placenta Endocrine Events that Orchestrate Reproduction

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Human placental lactogen (hPL) Human placental lactogen (hPL) Aids PRL in preparing the mammary glands for secretion Aids PRL in preparing the mammary glands for secretion Together with maternal growth hormone Together with maternal growth hormone hPL stimulates an increase in nutrients in the mother hPL stimulates an increase in nutrients in the mother The placenta also secretes  -endorphin The placenta also secretes  -endorphin Regulates appetite and mood during pregnancy Regulates appetite and mood during pregnancy Promote sense of well-being Promote sense of well-being Alleviate some of the discomfort associated with later months of pregnancy Alleviate some of the discomfort associated with later months of pregnancy Endocrine Events that Orchestrate Reproduction

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Stages of Childbirth 3 Stages of Childbirth First stage First stage Cervix enlarges Cervix enlarges Amniotic sac ruptures Amniotic sac ruptures Second stage Second stage Baby is forced out of the uterus and through the vagina Baby is forced out of the uterus and through the vagina Third stage Third stage Placenta or afterbirth is expelled Placenta or afterbirth is expelled Childbirth

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Milk Production Milk Production Milk production is triggered when infant sucks on the mother’s nipple Milk production is triggered when infant sucks on the mother’s nipple Stimulation leads to reflex release of oxytocin from the pituitary Stimulation leads to reflex release of oxytocin from the pituitary Oxytocin causes contraction of smooth muscles lining ducts of mammary glands Oxytocin causes contraction of smooth muscles lining ducts of mammary glands Suckling also stimulates release of prolactin, which continues milk production Suckling also stimulates release of prolactin, which continues milk production Endocrine Events that Orchestrate Reproduction

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Multiple Births Multiple Births Many mammals are multiparous Many mammals are multiparous Give birth to many offspring at one time Give birth to many offspring at one time Some are uniparous Some are uniparous Give birth only to one at a time Give birth only to one at a time Exceptions occur Exceptions occur Armadillos gives birth to four young, all male or all female Armadillos gives birth to four young, all male or all female Derived from one zygote Derived from one zygote Childbirth

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Monozygotic (Identical) Twins Monozygotic (Identical) Twins Derived from one zygote Derived from one zygote Have identical genomes Have identical genomes Dizygotic (Fraternal) Twins Dizygotic (Fraternal) Twins Derived from two zygotes Derived from two zygotes May not resemble each other any more than other siblings May not resemble each other any more than other siblings Childbirth

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Identical Twins Identical Twins One-third have separate placentas One-third have separate placentas Indicates blastomeres separated at an early stage Indicates blastomeres separated at an early stage Two-thirds share a placenta Two-thirds share a placenta Splitting occurred after formation of inner cell mass Splitting occurred after formation of inner cell mass Most have individual amniotic sacs Most have individual amniotic sacs A few share 1 amniotic sac and 1 placenta A few share 1 amniotic sac and 1 placenta Indicates that separation of the zygote occurred after day 9 of pregnancy, when the amnion has formed Indicates that separation of the zygote occurred after day 9 of pregnancy, when the amnion has formed These twins risk becoming conjoined (Siamese twinning) These twins risk becoming conjoined (Siamese twinning) Endocrine Events that Orchestrate Reproduction

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