In the placenta the embryonic and maternal vessels without actually communicating, are placed in intimate contact, which allows nutritive matter and oxygen to pass by endosmosis from the maternal vessels to those of the embryo. Figure 21 shows a human embryo at the beginning of the fifth week of pregnancy.
Fig. 22. Sagittal section of a primipara in the last month of pregnancy.
Duration of pregnancy. Birth. Pregnancy lasts from conjugation, which is synonymous with conception, till birth, that is about nine months (ten lunar months of four weeks). The embryo is then ready to separate from the maternal body (Fig. 22). By the act of birth it is expelled violently, bringing with it the umbilical cord and the placenta (Fig. 23). Immediately afterward the empty womb contracts strongly and gradually recovers its former size. The sudden interruption of its communications with the maternal circulation deprives the embryo, which has suddenly become a child, of its nutritive matter and oxygen.
Fig. 23. Sagittal section of frozen body of a woman in labor: the head of the child is engaged in the neck of the womb; the orifice of the neck of the womb (os uteri) is already fully dilated and the bag of waters commences to project from the vulva: it is formed by the former membranes of the egg and the decidua.
In order to avoid suffocation it is obliged to breathe atmospheric air immediately, for its blood becomes dark by saturation with carbonic acid, which irritates the respiratory nerve centers. The first independent act of the new-born child is, therefore, a nervous reflex determined by asphyxia, and is performed with the first cry. Soon afterward the infant begins to suck, so as not to die of hunger, while the umbilical cord, having become useless, shrivels up, and the placenta is destroyed (some animals eat it). The new-born infant is only distinguished from the embryo soon after birth by its breathing and crying.
We may, therefore, say that infancy, especially early infancy, is only a continuation of embryonic life. The transformations which the infant undergoes from birth to adult age are known to all. They take place more and more slowly, except at the relatively short period of puberty.
Formation of the sexual glands.—We must remember that at a very early embryonic period certain groups of cells are reserved to form later on the sexual glands. These cells are at first neither male nor female, but are undifferentiated; later on they become differentiated to form in certain individuals, called males, the testicles with their spermatozoa, and in others, called females, the ovaries with their eggs. On this differentiation depends the sex of the individual, and, according as it takes place in one way or the other, all the rest of the body develops with the correlative sexual characters of the corresponding sex (at first the external genital organs peculiar to each sex, then the beard in man, the breasts in woman, etc).