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Development Reading Guide
Fertilization occurs when the egg and sperm cells fuse. Within the ovum (or egg), the nuclei of the sperm and
ovum fuse, forming a diploid nucleus. This fertilized egg cell is called a zygote. Successful fertilization requires
living sperm in the oviduct on the day of, or shortly after, ovulation. On average of hundreds of millions sperm
are deposited in the female's vagina during intercourse. Yet, only one sperm is necessary to fertilize the egg. If an
egg is present when sperm are in the oviduct, many sperm will attach to the egg's surface. Each sperm cell has a
capsule at the forward end, called an acrosome, which contains enzymes that help the sperm cell penetrate the
protective coats of the egg. Once one sperm successfully contacts the plasma membrane of the egg, a barrier
forms on the egg's surface that prevents other sperm from entering.
1. What is fertilization? Where does it take place? What is the name of the cell that is formed in fertilization?
2. Why is it usually impossible for two sperm to fertilize a single egg?
Fertilization usually occurs in the region of the oviduct nearest the ovary. By about 36 hours after fertilization, a
process called cleavage has begun, during which the zygote undergoes a series of rapid mitotic divisions (Figure
33-9). The process of cleavage produces a non-hollow ball of cells called a morula..
By about the fifth day, cleavage has produced a hollow ball of cells that has reached the uterus with the help of
cilia in the oviduct. This ball of cells is called a blastula which consists of an outer layer of cells and a group of
cells inside the sphere Within the uterus, blastula’s cells secrete enzymes that stimulate implantation, the
imbedding of the blastocyst in the thickened endometrium that lines the uterus. The outer layer of the blastocyst
or trophoblast grows, extending into the endometrium and anchoring the blastocyst in place. The trophoblast
contributes to the development of membranes that will nourish and protect the embryo. The inner cell mass will
eventually form the organism itself.
3. What is the name of the rapid cell division that produces the morula?
4. How does the morula differ from the blastula?
5. What does the blastula do upon reaching the uterus?
Approximately three weeks after fertilization, a process called gastrulation takes place. Gastrulation forms three
different cell layers: ectoderm, endoderm, and mesoderm. The ectoderm (outer layer) forms the outer part of the
embryo's skin and the central nervous system. The endoderm (inner layer) forms the digestive tract and lungs. The
mesoderm (middle layer) forms most of the other organs. .During the first few weeks, membranes form that
protect and nourish the embryo. One membrane, the amnion, forms a fluid-filled sac. The amniotic membrane
protects the embryo from physical impact. The yolk sac membrane produces the first blood cells and is the source
of cells that eventually form gametes. The chorion is a third membrane that becomes the embryo's portion of the
placenta. Lastly, the allantois forms part of the umbilical cord that connects the embryo to the placenta.
6. What is the name of the process that produces different cells in the developing embryo?
7. What tissues form from the ectoderm, mesoderm and endoderm?
8. The trophoblast forms the amnion, yolk sac, chorion and allantois. What are the functions of these
membranes?
Soon after implantation, trophoblast cells and cells from the uterus form an important structure called the
placenta. The placenta develops inside the uterus and surrounds the embryo. This structure enables nutrients and
waste products to be transferred between the mother and developing baby (Figure 33-14). By the end of the third
month, the placenta is fully formed and functional. In the wall of the placenta, the mother's blood and baby's
blood remain isolated in separate circulatory systems. However, the mother's blood vessels release pools of blood
very close to the baby's blood vessels. Nutrients and waste products are able to diffuse back and forth through
interstitial fluid from one blood system to the other.
9. What is the function of the placenta?
10. Does the baby’s blood mix with the mother blood?
11. If the blood doesn’t mix, how do nutrients enter the baby’s blood and how do wastes get removed?
12. If nutrients, vitamins and minerals can diffuse into the baby’s blood, can alcohol, nicotine and other drugs
also diffusion from the mother to the baby?
About 12 days after fertilization, a streak appears on the disc-shaped gastrula. This streak will become the brain
and spinal chord. At the top of the streak, bulges appear that will become the brain. It is now called a neurula (3
weeks) 4 weeks after fertilization, the heart develops and begins to beat. It is now an embryo and it is the size of
a grain of rice
Neurula
Embryo
13. When does the brain and spinal cord begin to develop?
14. What event marks the beginning of the embryonic stage of development?
At about 8 weeks, all the main body parts are formed and bone cells are now being produced. The developing
human is called a fetus. At nine weeks, the fetus is approximately 3 cm long, about the length of a paper clip. It
has all of its organs and major body parts, including a head that is large in proportion to its body. By the end of
the first trimester, the fetus can move its arms and legs, turn its head, frown, and make sucking motions.
14. What event marks the beginning of the fetal stage of development?
During weeks 12 through 23, the period of the second trimester, the fetus increases in length to about 25 cm. By
the end of this period, the fetus weighs about 0.5 kg and has the face of an infant, complete with eyebrows and
eyelashes. At this point, the fetal heartbeat is easily detected and the fetus can be quite active. The mother can
usually feel this activity, which ranges from flutters to kicks and pokes. Because the fetus is now filling up much
of the space in the uterus, it curls forward into what is referred to as the "fetal position." From week 24 until
birth, the period of the third trimester, the fetus experiences rapid growth, becoming larger and gaining stronger
bones and muscles. The respiratory and circulatory systems undergo changes that will enable the baby to start
breathing air and perform other vital functions outside the mother's uterus when it is born. As it fills more and
more of the available space in the uterus, the fetus becomes less active.
15. The development of what two systems are vital in order for the baby to survive outside of the uterus?
At birth, the average baby is 40-50 cm long and weighs 2.7-4.5 kg (7-12 lbs), or about the size of a small
watermelon. The baby is born as a result of a series of strong, rhythmic contractions of the muscles of the
mother's uterus—a condition called labor. Once again, hormones of the endocrine system regulate the process.
During the final weeks of pregnancy, estrogen levels peak. This causes cells to form in muscles of the uterus that
serve as receptors for a hormone called oxytocin. As it reaches the final stages of development, the fetus and the
mother's pituitary gland start to secrete oxytocin. This secretion causes the muscles of the uterus to begin
contracting. These muscle contractions stimulate the secretion of more oxytocin, which causes more contractions,
in an increasingly intense cycle that results in the baby being forced out of the uterus
Within moments of entering its new world, the baby gasps its first breaths of air. Television and movies have not
painted a very accurate picture of what a newborn really looks like. Immediately after birth, most infants are
bluish in color and are covered with blood and mucus. Often the facial features are distorted from having been
forced through the narrow opening of the cervix and vagina. Usually, these distortions disappear within a few
hours, and of course each baby looks beautiful to its parents. Shortly after birth, additional contractions force the
placenta out of the uterus. This is sometimes called the afterbirth. The umbilical cord is tied off and cut. The
remnant of the cord will eventually dry up and fall off on its own, leaving a familiar scar known as the navel, or
belly button.
16. What is the name of the hormone that triggers labor?
17. How does a woman know that she is going into labor?
18. What organ contracts?
19. What organ must dilate?
20. What happens after the mother delivers the baby?