NOURISHING MEMBRANES 



497 



loose connective tissue. They thus form what is practically a single 

 mesodermal layer lined on its outer and inner surfaces with a simple epi- 

 thelium. They are 



ep. am; 



" 



shown in semi-dia- 

 gram in Figure 465. 



This membrane is 

 a part of the em- 

 bryo's body at this 

 time, and the fcetal 

 circulation extends 

 extensively into it as 

 a plexus of small 

 vessels in the meso- 

 dermal core. When 

 the embryo attains 

 a certain size, this 

 membrane is applied 

 by its chorionic sur- 

 face to the internal 

 epithelial surface of 

 the uterus and forms 

 some sort of adhesion 

 to it. Figure 465 

 shows a semi-dia- 

 grammatic represen- 

 tation of this in the 

 uterus of a cat, while 

 Figure 466 shows an 

 enlarged view of part 

 of this same section. 

 This relation contin- 

 ues to be maintained 

 in some parts of the 

 contact. But in 

 others a more inti- 

 mate association is 

 formed. This occurs in different regions in different mammals. In 

 man it is on one side of the uterus, and it is developed as follows (Fig. 

 467). 



The epithelium layer of the chorion is evaginated into a series of 

 branching villi, which push down (outward or distally with regard to 

 the chorion, proximally with regard to the uterine epithelium) into the 

 mucous layer of the uterus. As the villi advance, the surface of the 





ep. ch. 



conn. t. 



FIG. 466. Enlargement of part of Figure 465. ep.am., epi- 

 dermal layer of amnion ; mes.am., mesodermal layer of amnion ; 

 mes.ch., mesodermal layer of chorion; ep.ch., epithelial layer of 

 chorion merged with the simple epithelium that lines the uterus ; 

 conn.t., connective tissue ; bl., blood cells of embryo. X 520. 



