|extraembryonic membranes in reptiles made development on land possible.
a. If an embryo develops in water, the water supplies the oxygen and takes away the wastes.
b. The surrounding water prevents desiccation and provides a protective cushion.
c. For an embryo on land, these functions are performed by extraembryonic membranes.
2. Chick extraembryonic membranes develop from extensions of germ layers, which spread over yolk.
a. The chorion lies next to the shell and carries on gas exchange.
b. The amnion contains protective amniotic fluid that bathes a developing embryo.
c. The allantois collects nitrogenous wastes.
d. A yolk sac surrounds the remaining yolk that provides nourishment.
3. Humans also have these membranes; their function is modified for internal development.
a. The chorion develops into the fetal half of the placenta.
b. A yolk sac is the first site of blood cell formation.
c. Allantoic blood vessels become the umbilical blood vessels.
d. The amnion surrounds the embryo and cushions it with amniotic fluid.
4. Therefore, all chordate animals develop in water, either in bodies of water or within the amniotic fluid.
A. Embryonic Development
1. The First Week
a. Fertilization occurs in the upper third of the oviduct; cleavage begins as the embryo moves down this tube to the uterus.
b. By the time the embryo reaches the uterus on the third day, it is a morula.
c. By the fifth day, the morula is transformed into a blastocyst.
A blastocyst is a hollow ball of cells, resulting from cleavage.
The trophoblast is an outer single layer of cells, which later gives rise to the chorion.
The inner cell mass is the mass of cells from which the embryo, and eventually the fetus, will develop.
2. The Second Week
a. At end of the first week, an embryo begins the process of implantation in the wall of the uterus.
b. The trophoblast secretes enzymes to digest away some of the tissue and blood vessels of the uterine wall.
c. The trophoblast begins to secrete human chorionic gonadotropin causing the corpus luteum to be maintained.
d. As the week progresses, the inner cell mass detaches itself from the trophoblast, and two more extraembryonic membranes form: the yolk sac and amnion.
e. The yolk sac forms below the embryonic disk; with no nutritive function in humans, it is the site of blood cell formation.
f. As in chick development, a human amnion and its cavity are where the embryo (and then the fetus) develop.
g. In humans, amniotic fluid insulates against any thermal changes; it also cushions and protects the fetus from trauma.
h. Gastrulation occurs during this week resulting in the inner cell mass flattening into an embryonic disk.
1) The embryonic disk is composed of two cell layers: the ectoderm above and the endoderm below.
2) Once an embryonic disk elongates to form the primitive streak (similar to that found in birds), a third germ layer, the mesoderm, forms by invagination of the cells along the streak.
i. The trophoblast is reinforced by mesoderm and becomes the chorion.
3. The Third Week
a. The nervous system is the first organ system to become visually evident.
1) It appears as a thickening along the entire dorsal length of the embryo; invagination occurs as the neural folds appear.
2) When the neural folds meet at the midline, the neural tube is formed.
3) After the notochord is replaced by the vertebral column, the nerve cord is called the spinal cord.
b. The development of the heart begins in the third week and continues into the fourth.
1) The right and left heart tubes fuse; the heart begins pumping blood, although the chambers are not fully formed.
2) The veins enter this largely tubular heart posteriorly, and the arteries exit anteriorly.
3) Later the heart twists so that all of the major vessels are located anteriorly.
4. The Fourth and Fifth Weeks
a. A bridge of mesoderm (the body stalk) connects the caudal (tail) end of the embryo with the chorion, which has projections called chorionic villi.
b. The fourth extra embryonic membrane (the allantois) is contained in this stalk; its blood vessels become the umbilical blood vessels.
c. The head and tail then lift up, and the body stalk moves anteriorly by constriction.
d. Once this process is complete, the umbilical cord is fully formed.
e. Limb buds appear from which the arms and legs will later develop.
f. The head enlarges and the sense organs become more prominent.
g. Rudiments of the eyes, ears, and nose are evident.
5. The Sixth Through Eighth Weeks
a. The developing human becomes more humanlike in appearance.
b. As the brain develops, the head achieves its normal relationship with the body as a neck region develops.
c. The nervous system is developed well enough to permit reflex actions (e.g., the startle response to touch).
d. At the eighth week, the embryo is about 38 mm long and weighs no more than an aspirin tablet; all organs are established.
B. The Structure and Function of the Placenta
1. Providing gas, nutrient and waste exchange, the placenta begins formation once the embryo is fully implanted.
2. Chorionic villi are treelike extensions of the chorion.
a. Chorionic villi project into the maternal tissues.
b. Later, the chorionic villi disappear in all areas except where the placenta develops.
3. By the tenth week, the placenta is fully formed and has already begun to produce progesterone and estrogen.
a. Due to the negative feedback control by the hypothalamus and anterior pituitary, no new follicles mature.
b. They maintain the lining of uterus and there is no menstruation during pregnancy.
4. The chorionic villi are surrounded by maternal blood sinuses; the maternal and fetal blood do not mix.
5. Exchange of molecules between the fetal and maternal blood takes place across the walls of the chorionic villi.
6. CO2 and wastes move across from the fetus; O2 and nutrients flow from the maternal side.
7. The umbilical cord stretches between the placenta and the fetus.
8. Umbilical arteries transport CO2 and other waste molecules to the placenta for disposal; the umbilical vein transports O2 and nutrient molecules from the placenta to the rest of the fetal circulatory system.
9. Harmful chemicals can cross the placenta.
a. This is of particular concern during the embryonic period, when various structures are first forming.
b. Each organ has a sensitive period during which a substance can alter the normal development.
c. A pregnant woman who takes thalidomide tranquilizer between days 27 and 40 is likely to have an infant born with deformed limbs; after this time period, the infant is born normal.
C. Fetal Development and Birth
1. Fetal development (months 3–9) involves an extreme increase in size; the weight multiplies 600 times.
2. The genitalia appear in the third month and gender can be identified anatomically.
3. A fetus soon acquires hair, eyebrows, eyelashes, and nails.
4. Fine, downy hair (lanugo) covers the limbs and trunk; it later disappears.
5. The skin grows so fast that it wrinkles; a waxy vernix caseosa protects the skin from the watery amniotic fluid.
6. A fetus at first only flexes its limbs and nods its head; later it moves its limbs vigorously; a mother feels movements from the fourth month onward.
7. After 16 weeks, a fetal heartbeat is heard through a stethoscope.
8. A fetus born at 24 weeks may survive; the lungs are still immature and often cannot capture O2 adequately.
D. Stages of Birth
1. When the fetal brain matures, the hypothalamus causes the pituitary to stimulate the adrenal cortex so that androgens are released.
2. The placenta uses androgens as precursors for estrogens that stimulate the production of prostaglandin and oxytocin.
3. The hormones estrogen, prostaglandin, and oxytocin all cause the uterus to contract and expel the fetus.
4. The process of birth (parturition) has three stages: dilation of the cervix, birth of the baby, and expulsion of the placenta.