A HUMAN EMBRYO OP TWENTY-FOUR PAIRS OF SOMITES. 165 



CHORION. 



CHORIONIC MEMBRANE. 



The chorion forms a large vesicle, within the cavity of which lies the embryo. 

 It is composed of two continuous layers, an inner mesenchymal and an outer 

 epithelial. The outer surface of the chorionic membrane is covered with chorionic 

 villi over which the epithelium is continuous. On the side adjacent to the attach- 

 ment of the body-stalk the surface of the chorion is free from villi and here both 

 epithelium and mesenchymal layers are reduced in thickness. This side is entirely 

 collapsed and thrown into a large fold, as shown in text-figure 1. 



CHORIONIC VILLI. 



The chorionic villi are of variable size. The largest ones, which measure from 

 1.1 mm. to 1.3 mm. in height, belong to the chorion frondosum. At their bases 

 they are ordinarily smaller than farther out. One of this type is shown in plate 7, 

 figure 4. It divides dicrotomously into two large stalks. These again divide and 

 extend out almost to the end as large, stout trunks. The terminal branches are 

 short and form either rounded or pointed projections. Smaller villi are found in 

 between the larger ones, but they are few in number. 



HISTOLOGICAL STRUCTURE. 



Histologically the chorion and its villi are quite similar. The epithelium 

 which covers the underlying mesenchyma is composed of two distinct layers of 

 cells. The outer of these is made up of cuboidal cells without distinct boundaries. 

 The protoplasm of its cells is finely granular and vacuolated and stains deeply 

 with eosin. Its free surface is in most places covered with a prickle-process 

 border as described by Grosser 14 , giving very much the appearance of cilia. In 

 no place, however, could definite cilia be demonstrated. The nuclei are of irregular 

 rounded or oval contour and deeply staining. Beneath this layer is the basal layer 

 of cells, the so-called "Langhan's layer," which is visibly separable from the outer 

 layer. Its cells are also cuboidal, but the protoplasm in most cells is clearer, more 

 distinctly vacuolated, and not so deeply stained. The nuclei are of rounded shape 

 and are clearer than those of the outer layer. The basal surface of this layer rests 

 directly upon the mesenchyma, there being no basement membrane present. It is 

 irregular, owing to the varied shapes of the ends of the cells, which in places seem 

 to be drawn out into processes which unite with those of the mesenchymal cells. 



In connection with the syncytial layer are to be described the so-called giant 

 cells and cell islands. The former are masses of deeply staining protoplasm 

 containing several nuclei. They vary in size and in the number of nuclei con- 

 tained. As pointed out by Frassi 11 they are merely portions of the syncytium, 

 which in many cases are still attached to the epithelium. In my specimen such 

 attachments are readily observable. 



The cell-islands are masses of trophoblastic cells which, as stated by Grosser 13 , 

 are always attached to villi. They represent portions of the trophoblast which have 

 failed to become spread out over the villi. They are made up of large decidual-like 

 cells of polygonal shape and distinct boundaries. Their nuclei are usually shriveled 

 and their protoplasm is clear and vacuolated. In the centers of these cell-islands 

 are evidences of degeneration. The cells are much broken up, nuclei are very 



