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1. Describe the process of fertilization, including where and how it occurs. [18.1, pp.364-365, Fig. 18.2]
2. Describe a woman’s symptoms indicating that she is pregnant, and explain the basis of the
pregnancy test. [18.1, p.365]
3. Explain what development stages occur before implantation. [18.1, p. 365, Fig. 18.3]
State the four processes of development. [18.2, p.366]
List the three embryonic germ layers and what each gives rise to. [18.2, p. 366, Fig. 18.4, Table 18.1]
Explain the significance of the primitive streak and neurula. [18.2, pp. 366-367, Fig. 18.5]
Name the extraembryonic membranes stating the function of each. [18.2, p.368, Fig. 18.6]
Describe the structure and function of the placenta. [18.2, p.368]
Trace the path of blood in the fetus and between the fetus and the placenta. [18.2, p.368-369, Fig. 18.7]
Describe genetic testing of an embryo (amniocentesis). [Health Focus, pp.370-371, Fig. 18A]
Describe the weekly events of embryonic development and the monthly events of fetal development. [18.2,
p.372-374, Figs. 18.8-18.11]
Explain how male and female sex organs develop. [18.3, pp. 374-375, Fig. 18.12]
Describe the three stages of birth. [18.4, p.376, Fig. 18.13]
Describe the anatomy of the breast, and give the names and functions of the hormones that affect the breast.
[18.4, p.377, Fig. 18.14]
Describe the fluids produced by the breasts following birth. [18.4, p.377]
State three theories of aging. [18.45, p.378]
Describe the effects of aging on body systems and ways to prevent a decline in body functions. [17.5, pp.379380, Fig. 18.16]
Understand and use the bold-faced and italicized terms included in this chapter. [Understanding Key Terms,
18.1 Fertilization
Fertilization normally occurs in the upper third of the oviduct. Uterine and oviduct contractions may help transport
the sperm.
Fertilization occurs when a sperm and egg interact. The sperm must pass through the several layers of follicular
cells surrounding the egg, called the corona radiata. The egg cell also has its own plasma membrane, a vitelline
membrane, and the zona pellucida.
Sperm gain entry through a species-specific mechanism of contacting the vitelline membrane, then fusing with the
egg plasma membrane before the sperm nucleus enters the cell. Once the sperm and egg nuclei fuse, fertilization is
accomplished, and a zygote forms.
The zygote travels down the oviduct, begins to undergo cell divisions, and eventually implants in the
endometrium. It then secretes human chorionic gonadotropin (HCG), the presence of which is the basis for
pregnancy tests.
Image 0347l.jpg (Fig. 18.1)
Image 0348l.jpg (Fig. 18.2)
Image 0349al.jpg (Fig. 18.3)
Image 0349bl.jpg (Fig. 18.3)
Dynamic Human 2.0 CD-ROM
Mader ESP Modules Online
Case Studies Online
The Case of the Embryos Without Parents
Frozen Embryos
263 (Fig. 18.2)
264 (Fig. 18.3)
18.2 Development Before Birth
Processes of Development
Embryonic development of humans and all other animals includes the following processes:
Cleavage begins right after fertilization as the zygote divides and divides again. The size of the cell does not
increase during this stage.
Morphogenesis is the reshaping of the embryo as cells migrate to different areas.
Differentiation occurs as cells take on specific structures and then functions.
Growth accompanies cell division during embryonic development.
During cleavage, the mass of cells is referred to as a morula.
The blastula stage begins once the morula transforms into a hollow ball with an inner cell mass off to one
side. The inner cell mass becomes an embryonic disk composed of two germ layers: an upper ectoderm
and a lower endoderm.
During gastrulation, the bilayered disk elongates, and a primitive streak is seen at the midline of the
embryo. Some of the cells within the primitive streak invaginate, giving rise to a third germ layer, the
As differentiation continues throughout development, the three germ layers give rise to all other tissues
and organs of the body (see Table 18.1).
Mesoderm cells along the main axis give rise to a notochord, which is eventually replaced by the
vertebral column. Neural folds fuse to form a neural tube, which gives rise to the spinal cord and brain.
During neurulation, induction occurs, a process in which one tissue influences the development of
another. Induction probably requires the presence of certain chemicals that turn genes on.
Somites arise from the mesoderm. These become the muscles of the body and the vertebrae. A body
cavity called the coelom forms and is lined by mesoderm. The coelom becomes the thoracic and
abdominal cavities.
Extraembryonic Membranes
Extraembryonic membranes extend out beyond the embryo.
The amnion envelops the fetus in protective fluid.
The yolk sac is the first site of red blood cell formation, and part of this membrane eventually becomes a portion
of the umbilical cord.
The allantois contributes to the circulatory system, and its vessels become the umbilical blood vessels.
The chorion, the outermost membrane, contributes to the placenta.
Fetal Circulation
The chorion contributes the placenta on the fetal side, while uterine tissues supply the placenta on the maternal
side. Fetal and maternal blood do not mix. Instead, carbon dioxide and wastes diffuse from the fetal side of the
placenta, and oxygen and nutrients move from the maternal side to the fetal side. The umbilical cord carries gases
and nutrients to the fetus from the placenta. Harmful molecules can cross the placenta; this is especially damaging
during the embryonic period.
Path of Fetal Blood
Fetal circulation is different from adult circulation because the fetus does not breathe air. Blood passes
between the atria of the heart through an oval opening because not as much blood needs to travel to the
lungs. An arterial duct shunts blood between the pulmonary trunk and aorta for the same reason. Two
umbilical arteries lead to the placenta; one umbilical vein takes nutrients to the baby. The umbilical vein
joins a venous duct entering the vena cava. The persistence of the oval opening at birth is the most
common heart defect in newborns.
Embryonic Development
Embryonic development lasts from fertilization to the end of the second month of gestation, at which time all
organ systems have formed. Fetal development occurs from the third through the ninth months of pregnancy.
First Month
The zygote undergoes the morula stage, and the blastocyst implants in the uterine lining by the end of the
second week. The inner cell mass is present, and extraembryonic membranes are forming. The early
embryonic cells are called stem cells and may be useful in curing certain diseases. At the end of the first
month, organs are developed and the placenta is forming. Limb buds are present, and eyes, ears, and a
nose appear.
Second Month
Legs, arms, and digits are better formed, the head is large, and all internal organs have appeared. During
the first two months, the mother may experience nausea, breast tenderness, fatigue, and other symptoms.
At the end of two months, the embryonic stage is over.
Fetal Development
Fetal development extends from the third to the ninth month.
Third and Fourth Months
During the third and fourth months, the body increases in size, and epidermal refinements (eyelashes,
nipples) become apparent. Bone is replacing cartilage. During this time, it becomes possible to
distinguish males from females.
Fifth through Seventh Months
The mother can feel fetal movement. The fetus’s thin skin is covered with lanugo, and the eyelids open
At the end of seven months, the fetus can survive if born prematurely. Its lungs may lack surfactant,
however, putting the baby at risk.
Eighth and Ninth Months
During the last two months, the fetus grows greatly in size. It rotates its head down toward the cervix.
Image 0350l.jpg (Fig. 18.4)
Image 0351l.jpg (Fig. 18.5)
Image 0352l.jpg (Fig. 18.6)
Image 0353al.jpg (Fig. 18.7)
Image 0353bl.jpg (Fig. 18.7)
Image 0353cl.jpg (Fig. 18.7)
Image 0353dl.jpg (Fig. 18.7)
Image 0354al.jpg (Fig. 18A)
Image 0354bl.jpg (Fig. 18B)
Image 0354cl.jpg (Fig. 18C)
Image 0355l.jpg (Fig. 18.8)
Image 0356l.jpg (Fig. 18.9)
Image 0357l.jpg (Fig. 18.10)
Image 0358l.jpg (Fig. 18.11)
Dynamic Human 2.0 CD-ROM
Life Science Animations VRL 2.0
Animal Biology/Reproduction and Development/ Human
Development Before Implantation
Animal Biology/Reproduction and Development/ Human
Embryonic Development
Mader ESP Modules Online
Case Studies Online
Animals/Development/Early Development
Animals/Development/Human Development
Animals/Development/Hormones and Pregnancy
Artificial Womb
Hormones and Multiple Births
265 (Fig. 18.4)
266 (Fig. 18.5)
267 (Fig. 18.6)
268 (Fig. 18.7)
269 (Fig. 18A)
270 (Fig. 18.8)
18.3 Development of Male and Female Sex Organs
Gonads begin developing during the seventh week of gestation. Genes on the Y chromosome code for testes
In the absence of the Y chromosome, fetuses are female and develop a vagina, uterus, and ovaries.
Males and females have somewhat analogous development during various fetal stages.
Image 0359al.jpg (Fig. 18.12)
Image 0359bl.jpg (Fig. 18.12)
Image 0359cl.jpg (Fig. 18.12)
Image 0359dl.jpg (Fig. 18.12)
Image 0359el.jpg (Fig. 18.12)
Image 0359fl.jpg (Fig. 18.12)
271 (Fig. 18.12a)
272 (Fig. 18.12b)
18.4 Birth
Estrogen, prostaglandins, and oxytocin cause the uterus to contact and expel the fetus.
Stage 1
Stage 1 labor involves contractions that move the baby’s head downward, enhancing effacement and dilation of
the cervix. The amnion breaks, releasing amniotic fluid. This stage ends when the cervix is completely dilated.
Stage 2
Stage 2 labor has frequent contractions of longer duration. The mother experiences a desire to push. An episiotomy
is often performed to prevent tearing. The baby is pushed out during this stage.
Stage 3
Stage 3 is the delivery of the afterbirth.
Female Breast and Lactation
No milk is produced during pregnancy, but milk ducts and alveoli proliferate during that time, and the breasts
enlarge. Once the baby is delivered, the pituitary secretes prolactin, and milk is produced. Suckling of the baby at
the breast enhances milk production.
Image 0360l.jpg (Fig. 18.13)
Image 0361l.jpg (Fig. 18.14)
Dynamic Human 2.0 CD-ROM
Reproductive/Histology/Mammary Gland
Reproductive/Clinical Concepts/Breast Cancer
Mader ESP Modules Online
Case Studies Online
Animals/Development/Hormones and Pregnancy
Treatment of Critically Ill Newborns
Should You Need a License to Be a Parent?
273 (Fig. 18.13)
274 (Fig. 18.14)
18.5 Development After Birth
Development is a lifelong process into adulthood. After that period, aging occurs. Gerontology is the study of
Theories of Aging
Genetic in Origin
One theory of aging suggests that aging has a genetic basis. Cells can divide only so many times. As we
grow older, it may be that more cells age and die. Also, some cell lines may die before that maximum
number of cell divisions has been reached. In addition, offspring of long-lived people also tend to be
long-lived. Some people may have genes that code for efficient enzymes that remove free radicals,
causing the individuals to live longer.
Whole-Body Process
A second theory of aging suggests that a hormonal decline can affect many different organ systems. The
immune system no longer performs as well, which is perhaps why cancer is more prevalent in the elderly.
Aging may be due to the failure of a particular tissue type found throughout the body.
Extrinsic Factors
A third theory on aging suggests that years of poor health habits contribute most to aging. Osteoporosis is
a good example.
Effect of Age on Body Systems
Skin loses elasticity and becomes thinner with age, resulting in sagging and wrinkling. Fewer sweat
glands are present, so temperature regulation is less efficient.
Processing and Transporting
Deterioration of the cardiovascular system is the leading cause of death among the elderly. The heart
shrinks with age, and fatty deposits clog arteries. Low-cholesterol, low-fat diets slow degenerative
changes. Lungs lose some elasticity, so ventilation is reduced. A reduced blood supply to the kidneys
results in the kidneys becoming smaller and less efficient. The digestive tract may lose muscle tone but
still absorbs nutrients efficiently.
Integration and Coordination
Normal aging results in the loss of few nerve cells. Short- term memory may decline, but overall
cognitive skills remain. After age 50, there is a slow decline in the ability to hear higher frequencies, and
the lens of the eye does not accommodate as well. Loss of skeletal muscle mass is common but can be
controlled through exercise. Bone density declines, which can be slowed by adequate calcium intake and
The Reproductive System
Females undergo menopause and are no longer reproductive. In males, sperm production continues until
Aging Well
Good health habits started when young slow the aging process and contribute to a long, healthy life span.
Image 0362l.jpg (Fig. 18.15)
Image 0363l.jpg (Fig. 18.16)
Image 0364l.jpg (Fig. 18B)
Image 0365l.jpg (Fig. TA18.1)
Case Studies Online
Is Dr. Melissa Walker Too Old to Have a Baby?
275 (Fig. TA18.1)
New/Revised Text:
This was chapter 18 in the previous edition. The chapter has been reorganized. The human life cycle,
including mitosis and meiosis, now begins the chapter. The chapter ends with a discussion of chromosomal
inheritance abnormalities.
19.2 Mitosis contains a new topic Cytokinesis, which discusses cytokinesis and formation of a cleavage
19.4 Chromosomal Inheritance. The discussion of nondisjunction now precedes an expanded explanation
of nondisjunction, how it occurs, and its resulting chromosomal abnormalities. Down syndrome and other
syndromes caused by abnormalities in chromosome makeup follow the discussion of nondisjunction. The term
triplo-X syndrome has been changed to poly-X syndrome.
New Bioethical Focus: Cloning in Humans
New/Revised Figures:
19.1 Life cycle of humans; 19.8 Spermatogenesis and oogenesis; 19.9 Human karyotype preparation
New/Revised Tables:
19.1 Meiosis I Versus Mitosis; 19.2 Meiosis II Versus Mitosis; These new tables help summarize the
information given in the chapter.
Quality of life for the elderly
1. Ask the social director (or equivalent person) of a residential care facility for the elderly to visit the class and
discuss the care taken to insure a high quality of life. What are the factors that determine the quality of life for
an individual resident? (health, interests, exercise, religious faith, participation in social activities, etc.). What
chronic medical problems are most common among the elderly?
Planning Pregnancy
2. Students should read the Health Focus, “Preventing Birth Defects,” before coming to class. To introduce a
sense of lively competition, divide the class into males and females. Each group is to prepare two lists: (1)
decisions to be made before pregnancy occurs and (2) decisions to be made after pregnancy occurs. The
Health Focus can be a source of ideas, but other sources are also helpful. Complete sentences must be used;
the lists will be judged by their completeness. The group producing the longest and most sensible list receives
bonus points to add to their class score.
Advantages of Breast-Feeding
3. To help students make the best decisions regarding their future children, have students read the following
article: Newman, J. December 1995. How breast milk protects newborns. Scientific American 273(6): 76.
Breast milk is the most nutritionally sound choice for newborns and gives them antibodies that boost their
immune systems. Have students discuss the social pros and cons of breast-feeding. Some students in your
class may have direct experience with this topic. Alternatively, have a representative from the La Leche
League speak to your class about breast-feeding and breast milk.