360 EMBRYONIC MEMBRANES AND YOLK-SAC. [cHAP. 



a short statement of the relations between the maternal 

 and foetal tissues in the different varieties of placenta. 



The simplest known condition of the placenta is 

 that found in the pig (Fig. 118 II.). The papilla-like 

 foRtal villi fit into the maternal crypts. The villi (v) are 

 formed of a connective tissue core with capillaries, and 

 are covered by a layer of very flat epithelium (e) de- 

 rived from the subzonal membrane. The maternal 

 crypts are lined by the uterine epithelium (e), imme- 

 diately below which is a capillary plexus. The maternal 

 and foetal vessels are here separated by a double epi- 

 thelial layer. The same general arrangement holds 

 good in the diffused placentae of other forms, and in the 

 polycotyledonary placenta of the Ruminantia, but the 

 foetal villi in the latter (III.) acquire an arborescent form. 

 The maternal vessels retain the form of capillaries. 



In the deciduate placenta a much more compli- 

 cated arrangement is usually found. In the typical 

 zonary placenta of the fox and cat (IV. and V.), the 

 maternal tissue is broken up into a complete trabecular 

 meshwork, and in the interior of the trabeculse there 

 run dilated maternal capillaries (d'). The trabeculse 

 are covered by a more or less columnar uterine epi- 

 thelium (e), and are in contact on every side with foetal 

 villi. The capillaries of the foetal villi preserve their 

 normal size, and the villi are covered by a flat epithelial 

 layer (e). 



In the Sloth (VI.) which has a discoidal placenta the 

 maternal capillaries become still more dilated, and the 

 epithelium covering them is formed of very flat poly- 

 gonal cells. 



