X.] HISTOLOGY OF THE PLACENTA. 363 



DIAGRAMMATIC REPRESENTATIONS OF THE MINUTE STRUCTURE 

 OF THE PLACENTA. (From Turner.) 



F. the foetal ; M. the maternal placenta ; e. epithelium of cho- 

 rion ; e'. epithelium of maternal placenta ; d. foetal blood- 

 vessels ; d'. maternal blood-vessels ; v. villus. 



I. Placenta in its most generalized form. II. Structure of 

 placenta of a Pig. III. Of a Cow. IV. Of a Fox. V. Of a 

 Cat. 



VI. Structure of placenta of a Sloth. On the right side of 

 the figure the flat maternal epithelial cells are shewn in situ. 

 On the left side they are removed, and the dilated maternal vessel 

 with its blood-corpuscles is exposed. 



VII. Structure of Human placenta. In addition to the let- 

 ters already referred to, ds, ds. represents the decidua serotina of 

 the placenta ; t, t. trabeculae of serotina passing to the foetal villi ; 

 ca. curling artery ; up. utero-placental vein ; x. a prolongation of 

 maternal tissue on the exterior of the villus outside the cellular 

 layer e', which may represent either the endothelium of the 

 maternal blood-vessel or delicate connective tissue belonging to 

 the serotina, or both. The layer e' represents maternal cells 

 derived from the serotina. The layer of foetal epithelium cannot 

 be seen on the villi of the fully-formed human placenta. 



In the human placenta (VII.), as in that of Apes, 

 the greatest modification is found. Here the maternal 

 vessels have completely lost their capillary form, and 

 have become expanded into large freely communicating 

 sinuses (d f ). In these sinuses the foetal villi hang for 

 the most part freely, though occasionally attached to 

 their walls by strands of tissue (t). In the late stages 

 of fcetal life there is only one epithelial layer (e} be- 

 tween the maternal and fcetal vessels, which closely 

 invests the fcetal villi, but is part of the uterine tissue. 

 In the foetal villi the vessels retain their capillary form. 



