From the fourth week up to the end of the second month, the chorion grows rapidly; the villi also increase very greatly, both in number and in site; they give off numerous branches which embed themselves in the decidua and end in free thread-like or frequently clavate processes, the so-called " roots." As in their first appearance, so also during the later stages of their growth, the epithelial layer is always in advance of the connective-tissue core, the vilii presenting lateral processes or knobs caused by local thickenings of the epithelium, into which, later on, the vascular tissue penetrates.
In the course of the'third month, those villi which are in connection with the decidua reflexa begin to shrink, the blood-vessels of the part of the chorion from which they spfing undergoing at the same time a gradual diminution in size. The villi that are embedded in the decidua serotina, on the other hand, increase greatly in size and complexity, and ultimately form, as we shall see, the festal part of the placenta (cf. the diagrammatic figure 126).
In this way we get a distinction established between the chorion fron dosum, opposite the decidua serotina, which is very vascular, and beset with closely placed and richly branched villi; and the chorion hove op posite the decidua reflexa, which is a thin transparent membrane with no blood-vessels, and connected with the reflexa by scattered, slightly branched, inconspicuous villi.
Up to the end of the third month, the villi can be readily withdrawn from the crypts of the decidua in which they are lodged, and the foetal and maternal structures separated from one another; but after the placenta is once established, the connection between foetal and maternal elements becomes so intimate that complete separation is no longer practicable.
The Amnion.
The amnion, like the allantois, appears to develop in the human species in a somewhat aberrant manner, though it is very possible that further investigations will remove many of the apparent anomalies in its mode of origin. What is known of its early stages has already been explained in the preceding chapter.
The true amnion, with which alone we are now concerned, completely covers the dorsal surface of the embryo as early as the fourteenth day (cf. Fig. 105). Its condition at the middle of the third week is shown
in Fig. 107, where it is seen to be some distance away from the embryo. At the end of the fourth week, as at the corresponding stage of the rabbit or chick, the amnion (Figs. 111 and 112) invests the embryo very closely indeed.
During the second month the amnion grows rapidly, so as to leave a large space (the amnionic cavity) between itself and the embryo, a space occupied by the liquor amnii. The amnion, by its further growth, forms a sheath to the umbilical cord, and comes in very close contact with the chorion, from which it is usually separated by a small quantity of fluid, or else by a gelatinous membrana intermedia.
Liquor Amnii.—The liquor amnii, which occupies the space between the amnion and the embryo, varies much in quantity at different periods of gestation. It is apparently most abundant about the fifth or sixth month.. Its actual quantity varies also so much in different cases that it is difficult to fix its normal amount. When in excess—i.e., more than about 1 quarts—it constitutes the affection known as hydrops amnii.
The liquor amnii contains urea, especially during- the later months of gestation. It was formerly regarded as a nutritive fluid, but the re searches of Gusserow point strongly in favor of its being really excretory. Gusseroiv considers that in the early stages of development it is simply a transudation from the fetal vessels, but that later on it receives directly the urine discharged by the foetus.
The yolk-sac persists throughout the whole period of gestation in the fourth and fifth months. It is a roundish, white body about .39 inches in diameter, lying between the amnion and the chorion, and usually close to the edge of the placenta (cf. Fig. 126). It is connected by a long, slen der stalk with the umbilicus of the foetus, the stalk being—together with the allantoic stalk—invested by the sheath formed by the amnion.
At the end of gestation the yolk-sac is still present in the same situ ation close to the edge of the placenta ; it is rather smaller than before, measuring about .19 inches and very commonly adheres closely to the amnion.