604 , THE HUMAN EMBRYO. 



first appearance, so also during the later stages of tlieir growth, 

 the epithelial layer, or epiblast, is always in advance of the 

 mesoblastic connective-tissue core ; the villi presenting lateral 

 processes, or knobs, caused by local thickenings of the epithe- 

 lium, into which at a later stage the vascular connective tissue 

 penetrates. 



The villi are at first of uniform size over the whole surface of 

 the chorion (Fig. 188) ; but towards the end of the second month, 

 or early in the third, they begin to develop unequally. Opposite 

 the decidua serotina, or part of the uterine wall to which the 

 ovum is directly attached (Fig. 254, ds), the villi increase 

 greatly in size and in complexity, forming ultimately the fcetal 

 part of the placenta. Over the rest of the surface of the chorion, 

 opposite the decidua reflexa, dr, the villi, on the contrary, begin 

 to shrink ; the blood-vessels which supply them undergoing at 

 the same time a gradual diminution in size. 



In this way a distinction is established between the chorion 

 frondosum, opposite the decidua serotina, which is very vascular, 

 and beset with closely placed and richly branched villi ; and the 

 chorion laeve, opposite the decidua reflexa, which is a thin trans- 

 parent membrane, with no blood-vessels, and connected with the 

 decidua reflexa merely by a few scattered, slightly branched, and 

 inconspicuous villi. By the end of the fourth month, the villi 

 of the chorion laeve have almost completely disappeared, except 

 from a narrow fringe round the margin of the placenta, where 

 they persist until the close of gestation. 



Up to the end of the third month, the villi can be fairly 

 readily withdrawn from the crypts of the decidua in which they 

 are lodged, and the fcetal and maternal structures thus separated 

 from each other ; but, after the placenta is definitely established, 

 the connection between the foetal and maternal elements becomes 

 so intimate that complete separation is no longer practicable. 



The epithelium of the chorion frondosum undergoes impor- 

 tant changes during the later months of gestation. Of the two 

 layers of cells of which it consists from the first, the inner 

 or deeper layer becomes thickened in irregular patches, very 

 variable in number and in size ; the individual cells are also 

 very irregular, and show signs of degenerative changes. The 

 outer, or surface layer of epithelium undergoes more extensive 

 changes. The cell boundaries become lost, and the cell bodies 



