1. ASEXUAL AND SEXUAL REPRODUCTION
ASEXUAL AND SEXUAL REPRODUCTION
Copyright © 2009 Pearson Education, Inc.

2. Asexual reproduction results in the generation of genetically identical offspring
Asexual reproduction
One parent produces genetically identical offspring
Very rapid reproduction
Can proceed via
Budding
Fission
Fragmentation/regeneration
Student Misconceptions and Concerns
1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce.
Teaching Tips
1. Aphid life cycles also alternate between asexual and sexual reproductive strategies. Consider challenging your class to explain why aphids (and other animals) do this. In general, sexual reproduction is most common in times of stress, and may be related to overpopulation or environmental change.
Video: Hydra Budding
Copyright © 2009 Pearson Education, Inc.

3. Figure 27.1 Asexual reproduction of an aggregating sea anemone (Anthopleura elegantissima) by fission.

4. Sexual reproduction results in the generation of genetically unique offspring
Sexual reproduction involves the fusion of gametes from two parents
Resulting in genetic variation among offspring
Increased reproductive success in changing environments
Student Misconceptions and Concerns
1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce.
2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text.
Teaching Tips
1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal.
Copyright © 2009 Pearson Education, Inc.

5. Video: Hydra Releasing Sperm
Some organisms can reproduce
Asexually or
Sexually
Student Misconceptions and Concerns
1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce.
2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text.
Teaching Tips
1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal.
Video: Hydra Releasing Sperm
Copyright © 2009 Pearson Education, Inc.

6. Eggs
Figure 27.2A Asexual (left) and sexual (right) reproduction in the starlet sea anemone (Nematostella vectensis).

7. Some animals exhibit hermaphroditism
Some animals exhibit hermaphroditism
One individual with male and female reproductive systems
Easier to find a mate for animals less mobile or solitary
Student Misconceptions and Concerns
1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce.
2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text.
Teaching Tips
1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal.
Copyright © 2009 Pearson Education, Inc.

8. Figure 27.2B Hermaphroditic earthworms mating.

9. Sperm may be transferred to the female by
Sperm may be transferred to the female by
External fertilization
Many fish and amphibian species
Eggs and sperm are discharged near each other
Internal fertilization
Some fish and amphibian species
Nearly all terrestrial animals
Sperm is deposited in or near the female reproductive tract
Student Misconceptions and Concerns
1. Students do not often understand the costs and benefits of asexual and sexual reproduction. Consider discussing the advantages and disadvantages of each of these forms of reproduction. Encourage students to focus on the compromises involved in any adaptation. There is simply no one “best” way for all animals to reproduce.
2. Many students expect that hermaphroditic animals simply fertilize themselves. Although this may be common in some animals, the exchange of gametes between hermaphrodites also occurs, as noted in the text.
Teaching Tips
1. Many salamander species use spermatophores to transfer sperm from the male to the female. Spermatophores are reproductive structures produced by males during courtship. Sperm is deposited atop a gelatinous base attached to the substrate. The female moves over the spermatophore, removes some of the sperm from the cap, and stores the sperm in her reproductive tract (a spermatheca) until the time of egg deposition. Thus, sperm transfer is external but fertilization is internal.
Copyright © 2009 Pearson Education, Inc.

10. Eggs
Figure 27.2C Frogs in an embrace that triggers the release of eggs and sperm (the sperm are too small to be seen).

11. HUMAN REPRODUCTION
Copyright © 2009 Pearson Education, Inc.

12. Reproductive anatomy of the human female
Both sexes in humans have
A set of gonads where gametes are produced
Ducts for gamete transport
Structures for copulation
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Ectopic pregnancies occur when an embryo implants anywhere other than the uterus. Most frequently, ectopic pregnancies occur in the oviducts. However, the structure of the oviduct cannot accommodate the growth of a fetus. Surgical removal of the fetus, resulting in an abortion, is thus often required for the sake of the mother’s health. Many of those who believe that abortion is wrong in general nonetheless consider abortions in the case of ectopic pregnancy to be an exception, in that it may literally save the life of the mother.
2. Endometriosis is an inflammatory condition in which the endometrium spreads beyond the uterus. It can lead to painful menstrual cycles and infertility. The Endometriosis Association website, is a good resource for additional information.
Copyright © 2009 Pearson Education, Inc.

13. Reproductive anatomy of the human female
Ovaries contain follicles which
Nurture eggs
Produce sex hormones
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Ectopic pregnancies occur when an embryo implants anywhere other than the uterus. Most frequently, ectopic pregnancies occur in the oviducts. However, the structure of the oviduct cannot accommodate the growth of a fetus. Surgical removal of the fetus, resulting in an abortion, is thus often required for the sake of the mother’s health. Many of those who believe that abortion is wrong in general nonetheless consider abortions in the case of ectopic pregnancy to be an exception, in that it may literally save the life of the mother.
2. Endometriosis is an inflammatory condition in which the endometrium spreads beyond the uterus. It can lead to painful menstrual cycles and infertility. The Endometriosis Association website, is a good resource for additional information.
Animation: Female Reproductive Anatomy
Copyright © 2009 Pearson Education, Inc.

14. Oviduct Ovaries Follicles Corpus luteum Uterus Wall of uterus
Endometrium
(lining of uterus)
Cervix
(“neck” of uterus
Figure 27.3A Front view of female reproductive anatomy (upper portion).
Vagina

15. The uterus opens into the vagina through the cervix
Oviducts (Fallopian Tubes) convey eggs to the uterus where embryos develop
The uterus opens into the vagina through the cervix
The vagina
Receives, envaginates, the penis during sexual intercourse
Forms the birth canal
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Ectopic pregnancies occur when an embryo implants anywhere other than the uterus. Most frequently, ectopic pregnancies occur in the oviducts. However, the structure of the oviduct cannot accommodate the growth of a fetus. Surgical removal of the fetus, resulting in an abortion, is thus often required for the sake of the mother’s health. Many of those who believe that abortion is wrong in general nonetheless consider abortions in the case of ectopic pregnancy to be an exception, in that it may literally save the life of the mother.
2. Endometriosis is an inflammatory condition in which the endometrium spreads beyond the uterus. It can lead to painful menstrual cycles and infertility. The Endometriosis Association website, is a good resource for additional information.
Copyright © 2009 Pearson Education, Inc.

16. Egg
cell
Figure 27.3B Ovulation.
Ovary

17. Oviduct Ovary Uterus Urinary bladder Rectum Pubic bone Cervix Urethra
Figure 27.3C Side view of female reproductive anatomy (with nonreproductive structures in italic).
Vagina
Shaft
Prepuce
Clitoris
Glans
Labia minora
Anus
Labia majora
Vaginal opening

18. Reproductive anatomy of the human male
Testes (singular testis) produce
Sperm
Male hormones
Epididymis stores sperm as they develop further
Several glands contribute to semen
Seminal vesicles
Prostate
Bulbourethral
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Men in your class will be well aware of physiological changes in the scrotum associated with thermoregulation. When a man enters into cool water, the scrotum is pulled tight and the testes are held close to the body. During a warm shower or bath, the scrotum relaxes and the testes are held far away from the body.
2. Testicular cancer is the most common form of cancer in men between 15 and 35 years of age. Students can find information about testicular cancer and how to perform a self-exam at
Copyright © 2009 Pearson Education, Inc.

19. Urinary bladder Seminal vesicle (behind bladder) Prostate gland
Bulbourethral
gland
Urethra
Erectile tissue
of penis
Figure 27.4A Front view of male reproductive anatomy.
Scrotum
Vas deferens
Epididymis
Testis
Glans of
penis

20. Rectum Seminal vesicle Urinary bladder Vas deferens Ejaculatory duct
Pubic bone
Prostate gland
Erectile
tissue of
penis
Figure 27.4B Side view of male reproductive anatomy (with nonreproductive structures in italic).
Bulbourethral gland
Urethra
Penis
Vas deferens
Glans of
penis
Epididymis
Testis
Prepuce
Scrotum

21. Reproductive anatomy of the human male
During ejaculation
Sperm is expelled from the epididymis
The seminal vesicles, prostate, and bulbourethral glands secrete into the urethra
Semen is formed and expelled from the penis
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Men in your class will be well aware of physiological changes in the scrotum associated with thermoregulation. When a man enters into cool water, the scrotum is pulled tight and the testes are held close to the body. During a warm shower or bath, the scrotum relaxes and the testes are held far away from the body.
2. Testicular cancer is the most common form of cancer in men between 15 and 35 years of age. Students can find information about testicular cancer and how to perform a self-exam at
Copyright © 2009 Pearson Education, Inc.

22. The formation of sperm and egg requires meiosis
Spermatogenesis
Occurs in seminiferous tubules
Primary spermatocytes
Formed by mitosis
Divide by meiosis I to produce secondary spermatocytes
Secondary spermatocytes divide by meiosis II to produce spermatids
Round spermatids differentiate into elongate sperm
Mature sperm released into seminiferous tubule
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Ask students to explain why polar bodies are produced during oogenesis and why they have so little cytoplasm. Challenge your students to explain why polar bodies are not produced in spermatogenesis.
Copyright © 2009 Pearson Education, Inc.

23. (in prophase of Meiosis I)
Diploid cell
Differentiation and
onset of Meiosis I 2n
Primary spermatocyte
(in prophase of Meiosis I)
Meiosis I completed n n
Secondary spermatocyte
(haploid; double chromatids)
Meiosis II n n n n
Developing sperm cells
(haploid; single chromatids)
Figure 27.5A Spermatogenesis.
Differentiation
Sperm cells n n n n
(haploid)

24. The formation of sperm and egg requires meiosis
Oogenesis
Begins before birth—diploid cells start meiosis and stop
Each month about one primary oocyte resumes meiosis
A secondary oocyte arrested at metaphase of meiosis II is ovulated
Meiosis of the ovum is completed after fertilization
Student Misconceptions and Concerns
1. Students’ background knowledge of human reproductive biology is likely to be quite uneven. Furthermore, the embarrassment frequently associated with this topic makes it difficult for teachers to fairly assess what students know. The best advice may be to not assume too much.
2. Embarrassment with the subject of human reproductive biology may make open discussions uncomfortable for some students. Good clear textbook and media assignments that can be studied privately and opportunities to ask anonymous questions provide additional avenues to address sensitive content and questions.
Teaching Tips
1. Ask students to explain why polar bodies are produced during oogenesis and why they have so little cytoplasm. Challenge your students to explain why polar bodies are not produced in spermatogenesis.
Copyright © 2009 Pearson Education, Inc.

25. Diploid cell
in embryo 2n
Differentiation
and onset of
Meiosis I
Ovary 2n
Primary oocyte
(arrested in prophase
of Meiosis I; present
at birth)
Completion
of Meiosis I
and onset of
Meiosis II
Corpus
luteum
First
polar body n n
Secondary oocyte
Entry of sperm
triggers completion
of Meiosis II
(arrested at meta-
phase of Meiosis II;
released from ovary)
Growing
follicle
Figure 27.5B Oogenesis and the development of an ovarian follicle.
Second
polar body n
Mature follicle
Ovum
(haploid) n
Sperm
Ovulation
Ruptured follicle

26. Primary oocyte Secondary oocyte Ovum (haploid)
Diploid cell
in embryo 2n
Differentiation
and onset of
Meiosis I 2n
Primary oocyte
(arrested in prophase
of Meiosis I; present
at birth)
Completion
of Meiosis I
and onset of
Meiosis II
First
polar body n n
Secondary oocyte
Figure 27.5B Oogenesis and the development of an ovarian follicle.
Entry of sperm
triggers completion
of Meiosis II
(arrested at meta-
phase of Meiosis II;
released from ovary)
Second
polar body n
Ovum
(haploid) n

27. PRINCIPLES OF EMBRYONIC DEVELOPMENT
PRINCIPLES OF EMBRYONIC DEVELOPMENT
Copyright © 2009 Pearson Education, Inc.

28. Fertilization results in a zygote and triggers embryonic development
Embryonic development begins with fertilization
The union of sperm and egg
To form a diploid zygote
Student Misconceptions and Concerns
1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe?
Teaching Tips
1. The authors note in Module 27.9 that the processes of development in sea urchins and frogs are discussed because they exhibit fundamental details that are found in the development of most vertebrates. These similarities are a consequence of our shared ancestry and provide strong evidence of evolution.
2. Reproductive isolating mechanisms are generally classified into prezygotic and postzygotic categories. Module 27.9 discusses some of the ways that hybridization is prevented by species specific biochemical interactions between the sperm and the egg.
Copyright © 2009 Pearson Education, Inc.

29. Sperm are adapted to reach and fertilize an egg
Sperm are adapted to reach and fertilize an egg
Streamlined shape moves more easily through fluids
Many mitochondria provide ATP for tail movements
Head contains
A haploid nucleus
Tipped with penetrating enzymes in the acrosome
Student Misconceptions and Concerns
1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe?
Teaching Tips
1. The authors note in Module 27.9 that the processes of development in sea urchins and frogs are discussed because they exhibit fundamental details that are found in the development of most vertebrates. These similarities are a consequence of our shared ancestry and provide strong evidence of evolution.
2. Reproductive isolating mechanisms are generally classified into prezygotic and postzygotic categories. Module 27.9 discusses some of the ways that hybridization is prevented by species specific biochemical interactions between the sperm and the egg.
Copyright © 2009 Pearson Education, Inc.

30. Neck Middle Plasma membrane piece Head Tail Mitochondrion
(spiral shape)
Figure 27.9B The structure of a human sperm cell.
Nucleus
Acrosome

31. Fertilization results in a zygote and triggers embryonic development
Fertilization events
Sperm squeeze past follicle cells
Acrosomal enzymes pierce egg’s coat
Sperm binds
Sperm and egg plasma membranes fuse
Egg is stimulated to develop further
Egg and sperm nuclei fuse
Student Misconceptions and Concerns
1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe?
Teaching Tips
1. The authors note in Module 27.9 that the processes of development in sea urchins and frogs are discussed because they exhibit fundamental details that are found in the development of most vertebrates. These similarities are a consequence of our shared ancestry and provide strong evidence of evolution.
2. Reproductive isolating mechanisms are generally classified into prezygotic and postzygotic categories. Module 27.9 discusses some of the ways that hybridization is prevented by species specific biochemical interactions between the sperm and the egg.
Copyright © 2009 Pearson Education, Inc.

32. The sperm squeezes through cells left over from the follicle1
The sperm squeezes
through cells left
over from the follicle 2
The sperm’s
acrosomal
enzymes digest the
egg’s jelly coat 3
Proteins on the
sperm head bind to
egg receptors 4
The plasma membranes
of sperm and egg fuse
Sperm
Plasma
membrane 5
The sperm nucleus
enters the egg
cytoplasm
Nucleus
Acrosomal
enzymes
Acrosome
Sperm
head
A fertilization
envelope
forms 6
Receptor protein
molecules
Plasma
membrane
Figure 27.9C The process of fertilization in a sea urchin.
Sperm
nucleus
Vitelline
layer
Cytoplasm 7
The nuclei
of sperm and egg fuse
Jelly
coat
Egg
nucleus
Egg cell
Zygote nucleus

33. Figure 27.9A A human egg cell surrounded by sperm.

34. Cleavage produces a ball of cells from the zygote
Cleavage is a rapid series of cell divisions
More cells
Embryo does not get larger
Thus new cells are smaller in size
Student Misconceptions and Concerns
1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe?
Teaching Tips
1. Cleavage is largely a process of subdivision with no growth. This process is a bit like cutting up a pie into pieces. With every division, more pieces are produced, but the pieces are smaller and the pie itself does not increase in size. Similarly, cleavage increases the number of cells while decreasing their size.
2. Cleavage in humans is a very slow process, taking up to 12 hours for each division.
3. Cleavage results in an uneven distribution of cytoplasmic elements into the daughter blastomeres (embryonic cells produced by cleavage). Some students might benefit from this simple analogy: Imagine baking a pie that is filled with one can of apple pie filling and one can of cherry pie filling. The contents of each can are poured into opposite ends of the pie and are not mixed. When the pie is served, some pieces will contain only apples, some only cherries, and some combinations of both. This disproportionate division of pie contents is like the disproportionate division of cytoplasmic elements during cleavage.
Video: Sea Urchin Embryonic Development
Copyright © 2009 Pearson Education, Inc.

35. Zygote
2 cells
Figure Cleavage in a sea urchin.

36. Zygote
2 cells
4 cells
Figure Cleavage in a sea urchin.

37. Zygote
2 cells
4 cells
8 cells
Figure Cleavage in a sea urchin.

38. Zygote 2 cells 4 cells 8 cells Many cells (solid ball)
Figure Cleavage in a sea urchin.
Many cells
(solid ball)

39. Blastula (hollow ball) Cross section of blastula
Zygote
2 cells
4 cells
8 cells
Figure Cleavage in a sea urchin.
Blastula
(hollow ball)
Cross section
of blastula
Many cells
(solid ball)
Blastocoel

40. Gastrulation produces a three-layered embryo
Gastrulation
Cells migrate
The basic body plan of three layers is established
Ectoderm outside—becomes skin and nervous systems
Endoderm inside—becomes digestive tract
Mesoderm in middle—muscle and bone
A rudimentary digestive cavity forms
Student Misconceptions and Concerns
1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe?
Teaching Tips
1. You might wish to reflect on the somewhat-famous quote of Lewis Wolpert, who in 1986 said, “It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” The development and arrangement of the basic embryonic layers (ectoderm : skin and nervous system; mesoderm : muscle and bone; and endoderm : digestive tract) establishes the basic body plan.
2. Gastrulation establishes the basic body plan by positioning the future systems of the body in relationship to each other. This essential element of gastrulation is a crucial event that is a prerequisite for later developmental events (and thus is the basis of the quote directly above).
3. The colors used in the text to depict the three layers of embryonic tissue are standards used in embryology. Blue represents ectoderm and its derivatives, red represents mesoderm and its derivatives, and yellow represents endoderm and its derivatives.
4. Neural crest cells are often referred to as the fourth layer of embryonic tissue because these cells give rise to a variety of tissues in diverse locations in the body. Neural crest cells and their derivatives are not addressed in Chapter 27.
Copyright © 2009 Pearson Education, Inc.

41. Blastula Animal pole (end of cleavage) Blastocoel Vegetal pole
Gastrulation
(cell migration)
Formation of a
simple digestive
cavity
Blastocoel
shrinking
Blastopore
Figure Development of the frog gastrula.
Gastrula
(end of gastrulation)
Ectoderm
Mesoderm
Simple
digestive
cavity
Endoderm

42. Animal pole Blastula (end of cleavage) Blastocoel Vegetal pole
Gastrulation
(cell migration)
Figure Development of the frog gastrula.
Formation of a
simple digestive
cavity
Blastocoel
shrinking
Blastopore

43. Gastrulation (cell migration) Formation of a simple digestive cavity
Blastocoel
shrinking
Blastopore
Ectoderm
Gastrula
(end of gastrulation)
Mesoderm
Figure Development of the frog gastrula.
Endoderm
Simple
digestive
cavity

44. Organs start to form after gastrulation
Organs develop from the three embryonic layers
Stiff notochord forms the main axis of the body
Later replaced by the vertebral column
Neural tube develops above the notochord and will become the
Brain
Spinal cord
Student Misconceptions and Concerns
1. The descriptions of basic development address questions that students may have never thought to ask. How do we get so many cells (trillions!) in our adult bodies? How do basic tissues, organs, and organ systems form? What mechanisms permit the coordinated development of the body? Before beginning this chapter, consider challenging your students with some of these fundamental questions: How did you get a heart, a brain, and ears? What determined when you were ready to be born? How did you get nutrients and oxygen before you could eat or breathe?
2. Many students do not realize that their nervous system is hollow, and do not consequently relate the formation of the neural tube to the structure of a fully developed adult nervous system. While discussing formation of the neural tube, you may want to anticipate the subject matter of Chapter 28 by emphasizing that the fluid-filled ventricles of the brain and the central canal of the spinal cord are essentially empty spaces.
Teaching Tips
1. The notochord functions as a sort of scaffolding upon which the embryo develops. In the lancelet (amphioxus), the notochord is the adult skeletal structure along the long axis of the body.
2. The alignment of the notochord and neural tube is not accidental. As discussed in Module 27.13, cellular communication between these two developing structures in the form of inductive events permits the alignment of related structures.
Video: Frog Embryo Development
Copyright © 2009 Pearson Education, Inc.

45. HUMAN DEVELOPMENT
Copyright © 2009 Pearson Education, Inc.

46. The embryo and placenta take shape during the first month of pregnancy
Human fertilization occurs in the oviduct
Cleavage produces a blastocyst (blastula)
Inner cell mass becomes the embryo
Trophoblast
Outer cell layer
Attaches to the uterine wall
Forms part of the placenta
Student Misconceptions and Concerns
1. Students often think that maternal and fetal blood merge together. Few students have an accurate understanding of the relationship of fetal and maternal blood in the placenta.
2. The extraembryonic membranes surrounding a human embryo are also rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts.
Teaching Tips
1. All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees!
Copyright © 2009 Pearson Education, Inc.

47. Cleavage starts Fertilization of ovum Ovary Oviduct Secondary oocyte
Blastocyst
(implanted)
Figure 27.15A From ovulation to implantation.
Ovulation
Endometrium
Uterus

48. Endometrium Inner cell mass Cavity Trophoblast
Figure 27.15B Blastocyst (6 days after conception).
Trophoblast

49. Multiplying cells of trophoblast Trophoblast Uterine cavity
Endometrium
Blood vessel
(maternal)
Future embryo
Multiplying cells
of trophoblast
Future
yolk sac
Trophoblast
Figure 27.15C Implantation under way (about 7 days).
Uterine cavity

50. The embryo and placenta take shape during the first month of pregnancy
Embryo and placenta take shape
Placenta
Site of
Gas exchange—from mother to embryo
Nutrient exchange—from mother to embryo
Waste exchange—from embryo to mother
Student Misconceptions and Concerns
1. Students often think that maternal and fetal blood merge together. Few students have an accurate understanding of the relationship of fetal and maternal blood in the placenta.
2. The extraembryonic membranes surrounding a human embryo are also rarely understood by college students outside of the health sciences. These extraembryonic membranes can be viewed as analogous to the life-support systems used by astronauts.
Teaching Tips
1. All vertebrates develop in a fluid environment. Fish and amphibians typically lay their eggs in water. Amniotes (reptiles and mammals) are defined by the presence of an amniotic sac, a self contained aquatic system that surrounds the embryo with water inside the egg. The evolution of the amniotic egg eliminated the need to reproduce near water, freeing amniotes to reproduce underground, in deserts, and in trees!
Copyright © 2009 Pearson Education, Inc.

51. Amnion Mesoderm cells Chorion Yolk sac Amniotic cavity
Figure 27.15D Embryonic layers and extraembryonic membranes starting to form (9 days).
Yolk sac

52. Chorion Chorionic villi Amnion Embryo: Ectoderm Allantois Mesoderm
Endoderm
Yolk sac
Figure 27.15E Three-layered embryo and four extraembryonic membranes (16 days).

53. Placenta Mother’s blood vessels Allantois Amniotic cavity Yolk sac
Amnion
Embryo
Figure 27.15F Placenta formed (31 days).
Chorion
Chorionic
villi

54. Video: Ultrasound of Human Fetus 1 Video: Ultrasound of Human Fetus 2
27.16 Human development from conception to birth is divided into three trimesters
First trimester
Time of most radical change for mother and embryo
Embryo forms—looks like other vertebrate embryos
Extraembryonic membranes form
All major organ systems are established
At 9 weeks after fertilization, now called a fetus
Can move its arms and legs
Starts to look distinctly human
Teaching Tips
1. Morning sickness typically occurs during the first trimester of pregnancy and subsides during the second. However, a great deal of variation in the timing and degree of symptoms has been observed. Unfortunately, the precise cause of morning sickness remains unknown.
2. Most human babies weigh 3–4 kilograms (kg) at birth. Birth weights much larger or smaller than this are associated with increased mortality. This is an example of stabilizing selection, which is discussed in Module
Video: Ultrasound of Human Fetus 1
Video: Ultrasound of Human Fetus 2
Copyright © 2009 Pearson Education, Inc.

55. Figure 27.16A Timeline of Human Fetal Development.
5 weeks (35 days)

56. Figure 27.16B Timeline of Human Fetal Development.
9 weeks (63 days)

57. Human development from conception to birth is divided into three trimesters
Second trimester
Increase in size
Refinement of human features
At 20 weeks
About 19 cm long (7.6 in.)
Weighs about 0.5 kg (1 lb.)
Teaching Tips
1. Morning sickness typically occurs during the first trimester of pregnancy and subsides during the second. However, a great deal of variation in the timing and degree of symptoms has been observed. Unfortunately, the precise cause of morning sickness remains unknown.
2. Most human babies weigh 3–4 kilograms (kg) at birth. Birth weights much larger or smaller than this are associated with increased mortality. This is an example of stabilizing selection, which is discussed in Module
Copyright © 2009 Pearson Education, Inc.

58. Figure 27.16C Timeline of Human Fetal Development.
14 weeks (98 days)

59. Figure 27.16D Timeline of Human Fetal Development.
20 weeks (140 days)

60. Human development from conception to birth is divided into three trimesters
Third trimester
Time of rapid growth
Circulatory and respiratory systems mature
Muscles thicken and skeleton hardens
Ends with birth
Babies born at start of third trimester (28 weeks) may survive with extensive medical care
Teaching Tips
1. Morning sickness typically occurs during the first trimester of pregnancy and subsides during the second. However, a great deal of variation in the timing and degree of symptoms has been observed. Unfortunately, the precise cause of morning sickness remains unknown.
2. Most human babies weigh 3–4 kilograms (kg) at birth. Birth weights much larger or smaller than this are associated with increased mortality. This is an example of stabilizing selection, which is discussed in Module
Copyright © 2009 Pearson Education, Inc.

61. Figure 27.16E Timeline of Human Fetal Development.
At birth (280 days)