MAMMALIA. 259 



/'. the foetal; M. the maternal placenta ; e, epithelium of chorion; e'. epithelium 

 of maternal placenta; d. fcetal blood-vessels; d' . maternal blood-vessels; v. villus. 



A. Placenta in its most generalized form. 



B. Structure of placenta of a Pig. 



C. Structure of placenta of a Cow. 



D. Structure of placenta of a Fox. 



E. Structure of placenta of a Cat. 



F. 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. 



G. Structure of Human placenta. In addition to the letters already referred to 

 ds, ds. represents the decidua serotina of the placenta; /, t. trabeculce of serotina 

 passing to the fcetal villi; ca. curling artery ; up. utero-placental vein; x. a prolonga- 

 tion 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 con- 

 nective tissue belonging to the serotina, or both. The layer e' represents maternal 

 cells derived from the serotina. The layer of fcetal epithelium cannot be seen on the 

 villi of the fully-formed human placenta. 



is found in that the maternal vessels have completely lost their capillary 

 form, and have become expanded into large freely communicating sinuses 

 (d'). In these sinuses the fcetal villi hang for the most part freely, though 

 occasionally attached to their walls (/). In the late stages of fcetal life there 

 is only one epithelial layer (/) between the maternal and fcetal vessels, which 

 closely invests the fcetal villi, but, as shewn by Turner and Ercolani, is part 

 of the uterine tissue. In the foetal villi the vessels retain their capillary 

 form. 



Evolution of the Placenta. 



From Owen's observations on the Marsupials it is clear that 

 the yolk-sack in this group plays an important, if not the most 

 important part, in absorbing the maternal nutriment destined 

 for the fcetus. The fact that in Marsupials both the yolk-sack 

 and the allantois are functional in rendering the chorion 

 vascular makes it d priori probable that this was also the case in 

 the primitive types of the Placentalia, and this deduction is 

 supported by the fact that in the Rodentia, Insectivora and 

 Cheiroptera this peculiarity of the fcetal membranes is actually 

 found. In the primitive Placentalia there was probably present a 

 discoidal allantoic region of the chorion, from which simple fcetal 

 villi, like those of the pig (fig. 161 B), projected into uterine 

 crypts ; but it is not certain how far the umbilical part of the 

 chorion, which was no doubt vascular, may also have been 



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