Embryonic Development

About 36 hours after fertilization, zygote begins to devide by mitosis. The zygote continues to devide, forming a hollow ball of cells called the blastula. At this stage a major transformation occurs. The inner cell mass develops three layers, which eventually develop into the variousorgans of the body. This process is called gastrulation. The blastula then moves down the oviduct and into the uterus where it is now called a blastocyst. Only one part of the blastocyst will develop into embryo. An unborn child is called an embryo during the first eight weeks of its development. Afterward it is referred to as a fetus. The outer blastocyst cells, called the trophoblast, are not part of the embryo.

Six days after fertilization, the blastocyst attaches itself to the lining of the uterus, a process called implantation. From the trophoblast cells, four protective membranes are formed. They are the yolk sac, allantois, amnion, and chorion. The yolk sac has no yoik, as it does in birds, and serves no major function in the human embryo. The allantois functions in the formation of fetal blood vessels and gives rise to the umbilical cord. The umbilical cord, which contains two arteries and one vein, is the chief connection with the mother's uterus and permits the em­bryo to float in fluid. The amnion forms a sac, which fills with fluid. This amniotk fluid cushions the embryo and keeps it moist. The outermost membrane, the chorion, forms fingerlike projections, called villi, that attach to the lining of the uterus. Blood vessels from the ailantois run through the villi.

Together the chorion and allantois form the placenta. The placenta is a mass of tissue that secretes estrogen and progester­one, which help maintain a thickened, blood-enriched uterine lining. The placenta also serves as the point of exchange of substances between mother and fetus. The exchange takes place through the umbilical cord by which the fetus is attached to the placenta. However, the connection between the circulatory sys­tems of the mother and child is not a direct one. The mother's body carries out the processes of digestion, excretion, and respi­ration for the developing child. Food and oxygen pass from the mother's bloodstream into the capillaries within the villi of The placenta. The food and oxygen then pass into adjacent capillar­ies belonging to the fetus and, thus, into the main bloodstream of the fetus. Waste products pass from the bloom If the fetus to the blood of the mother in the same manner.

The nine-month pregnancy is commonly divided into three three-month periods called trimesters. During the first trimes­ter, unorganized cells become organized into vital organs. In the second trimester, the organs and body features become more refined. After 25 weeks, development is complete. During the final trimester, most growth in body size takes piace. A chiid born at this time would be premature but. with proper care, it would have a good chance of surviving outside its mother's body.

Childbirth

After approximately 266 days inside its mother's body, the child is ready for birth. Shortly before birth the mother's body under­goes changes that prepare it for labor, the process of literally forcing the child our of her body. Contractions of uterine mus­cles become progressively stronger due to a decrease in proges­terone, an increase in estrogen, and the release of the hormone oxytocin by the hypothalamus. Oxytocin speeds up contractions of the uterus. The cervix relaxes and widens from about 0.5 cm to 10 cm. As the baby enters the birth canal, the amnion breaks, and amniotic fluid escapes. Muscle contractions become very strong and finally push the infant out of the mother's body. The passing of the child from the uterus into the external environ­ment is called delivery.

After delivery the umbilical cord is cut and tied to prevent a loss of blood through the umbilical blood vessels. The child must now support its own life processes. During the gestation period, fetal lungs do not operate. At birth carbon dioxide from cellular activity quickly builds up in the child's blood. This buildup triggers the respiratory center in the brain to take in oxygen, and the child begins breathing on its own.

Fetal blood follows a path that bypasses the lungs. Freshly oxygenated blood flows from the placenta to the heart and deox-ygenated fetai blood flows through the heart in a path which leads it back to the placenta where it is oxygenated once again. Two openings in the fetal heart make this circulation possible.