FCETAL MEMBRANES. 



109 



In some Mammals (e.g., pig, horse, hippopotamus, camel) there develops a 

 more intimate relation between the chorion and the uterine mucosa. In the 

 pig, for example, the chorionic vesicle becomes somewhat spindle-shaped, and, 

 except at its tapering ends, its surface is closely applied to the surface of the 

 uterine mucosa. On that portion of the chorion which is in contact with the 

 uterine mucosa small elevations or projections develop and fit into correspond- 

 ing depressions in the mucosa. These projections involve the epithelial 

 layer (ectoderm) of the chorion and the adjacent connective tissue (mesoderm) 

 (Fig. 102) . Furthermore, the chorionic epithelial cells and the uterine epithelial 



Blood vessel in 

 uterine mucosa 



FIG. 103. From section through wall of uterus and chorion of a pig, showing close relationship 

 between the epithelium of the uterus and that of the chorion. Photograph. 



cells acquire very intimate relations in that the ends of the former become 

 rounded and fit into depressions in the ends of the latter (Fig. 103). 



The allantois and allantoic vessels in the pig afford a good example of the 

 transition from the respiratory and excretory functions which they almost ex- 

 clusively possess in Reptiles and Birds, to the additional nutritional function of 

 these vessels in Mammals. The allantoic sac becomes large and applies itself 

 to the inner surface of the chorion, so that the blood vessels of the allantois also 

 grow into and ramify in the mesodermal layer of the chorion. This brings the 

 allantoic (umbilical) blood vessels containing the foetal blood closer to the uterine 

 vessels containing the maternal blood. The two sets of vessels never come in 

 contact, however, being always separated by the chorionic and uterine epithe- 



