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PROCESS OF FERTILIZATION
After the process, described in the following paragraph, whereby the embryo becomes implanted in the wall of the uterus, the three layers of primitive germ cells-the ectoderm, the endoderm, and the mesoderm become differentiated. Eventually the nervous system, the sense organs, mouth cavity, and skin will develop from the ectoderm; from the endoderm will come the digestive and respiratory systems; and from the mesoderm, the muscular, skeletal, circulatory, excretory, and reproductive systems.
About the seventh or eighth day after fertilization, the blastocyst comes into direct contact with the prepared wall of the uterus, the endometrium, and adheres to it. The cells of the trophoblast apparently produce enzymes that dissolve the maternal tissue to permit entry of the blastocyst, and by the twelfth day after fertilization, the embryo has buried itself completely within the endometrium. Little finger-like protrusions, the chorionic villi, grow from the chorion outward into the maternal tissue. Eventually these villi limit themselves to the ultimate point of junction between embryo and uterus.
During embryonic life, peripheral membranes form and then extend beyond the region in which the embryo itself develops. They serve as a means of obtaining food and oxygen, and as an avenue for the elimination of wastes from the embryo. During the first fourteen days of gestation, the embryo has not yet developed a functioning circulatory system, and food is obtained primarily by osmosis. Since the peripheral membranes are not incorporated within the body of the embryo and are discarded at the time of birth, they are called extraembryonic or fetal membranes. These membranes including the yolk sac, chorion, amnion, body stalk, and allantois begin developing about the second or third week of embryonic life.
Although virtually no yolk accumulates in the human ovum, a yolk sac is formed just as if a yolk existed. This yolk sac, which is lined with endodermal cells, is the primary material from which the primitive digestive tract is made. As the embryo develops, there is a progressive constriction of the yolk sac until it is connected to the embryo only by a thread-like structure called the yolk stalk, which ultimately becomes incorporated into the umbilical cord.
The amnion is a thin, transparent, tough membrane composed of a layer of ectodermal cells with an external covering of mesodermal cells. The amniotic cavity surrounded by this membrane appears before the body of the embryo has taken a definite shape, and is filled with a clear watery fluid called the amniotic fluid. The developing embryo is suspended in this fluid by its umbilical cord. The amniotic fluid has several important functions: it equalizes the pressure about the embryo, thus protecting it from jolts and mechanical injuries; it prevents the embryo from forming adhesions to the amnion that could result in malformations; it permits changes in fetal posture; and it acts as a hydrostatic wedge to facilitate childbirth by helping to dilate the neck of the uterus. In about the fifth month of pregnancy, the fetus usually begins to swallow some of the amniotic fluid. The infant's first bowel movements, consequently, are a discharge of this swallowed liquid. The baby's respiratory passages may also have to be cleared of some of the fluid after birth in order for normal respiration to begin.
The allantois, which ultimately constitutes part of the umbilical cord, is a tubular division of the posterior part of the yolk sac. During the development of the embryo, furthermore, it fuses with the chorion (described below) in the formation of the placenta. Functionally, the allantois of itself has no great importance except that it acts as a rudimentary umbilical cord in the early weeks of gestation.
The chorion, the outermost extra-embryonic membrane, completely surrounds the embryo. It is composed of two layers of epithelial cells: an outer ectoderm and an inner mesoderm. The most important role played by the chorion is in the formation of the placenta.
The placenta is formed by the interlocking of the decidua basalis (the portion of the uterine mucosa or endometrium directly underlying the chorionic vesicle) with the chorion frondosum, which is the external surface of the chorion that is covered with villi (finger-like projections). The placenta, comprised of uterine tissue and its interwoven villi, serves as a special organ of interchange between embryo and mother. The growth of the placenta is fairly rapid until about the fifth month of pregnancy. It has then reached its greatest relative size, which is approximately one-half of the internal surface of the uterus. The villi of the placenta are kept steeped in fresh maternal blood that enters the placental spaces about the villi by means of small blood vessels. As the blood drains back into the veins of the uterus, it is replaced by fresh blood from the uterine arteries.
From the beginning of its development, the fetal circulation of blood is a closed circuit. At no time during any stage of normal pregnancy is there any intermingling of maternal with fetal blood. The intermingling of the two systems can occur only in the case of injury to some portion of the placenta. The maternal and fetal blood both circulate within the placenta, but are kept separated by the walls of the umbilical blood vessels. All interchange between the two systems is by diffusion and absorption. A red blood cell, which is only about 1/30,000 of an inch in size, is too large to pass through the openings of the walls, yet chemicals and food bodies are small enough to penetrate the walls with no difficulty. The fetal blood absorbs food and oxygen, and it eliminates carbon dioxide and other metabolic waste products. These are taken into the mother's blood and eventually eliminated by her as waste products. Although the cellular barrier between the two blood systems generally prevents the passage of bacteria and other disease germs, some substances antibiotics, certain viruses, and some disease germs such as the Treponema pallidum, which causes syphilis are capable of crossing the barrier. The exact direction in which maternal blood flows within the placenta is not yet determined, but it is generally thought to pass through the marginal sinus and return to the uterine veins in the decidua basalis.
During the fifth week of pregnancy, the umbilical cord is formed. It is composed of the yolk stalk and its vitelline blood vessels, the allantois, and the umbilical blood vessels. The fully developed cord is about twenty inches long, which is also the average length of a full-term fetus.
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Pharmacy Information
SEXUALITY DEFINED: PAGAN-POLYTHEISTIC INFLUENCES - ROMAN SEXUALITY
Although there is reasonable agreement among most authorities regarding the characteristics of Greek sexuality, Roman customs and values are a matter of considerable debate. Conventional wisdom holds that the Roman society was as sexually promiscuous as the Greek; and it is popularly believed that the collapse of the Roman Empire was due, in no small part, to the sexual excesses of its citizens (Canter, 1963). These views have been challenged, however, on the grounds that the sexual excesses of Roman society have been greatly exaggerated and that the accounts of these events are biased and inaccurate (Bullough, 1976). Whatever the actual case, certainly Roman culture was more complicated and multidimensional than Greek society. The Roman Empire endured for a relatively long period of time; and, whereas the Greeks retained their beliefs and customs, Roman culture changed in the light of the changing empire and in the face of continuous exposure to other cultures.
The Romans, of course, shared the polytheistic religion of the Greeks, as well as the sexuality of their deities. Thus, they shared the Grecian religious approval of sex. As Grimal states:
To love was to obey the gods and achieve one of the requisites of the human condition. Chastity could be required by religious rites in certain cases, but it was not a good thing in itself; not even a desirable thing; it was rather an impairment of what was good and desirable for among the gifts of the gods to man, love is always to be found. [Grimal, 1967, p. xiii]
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