FCETAL NUTRITION: THE PLACENTA 451 



formed embraces about one-half of the circumference of the 

 blastodermic vesicle. With the union of the allantois and diplo- 

 trophoblast, the circulation in the decidua reflexa decreases, 

 and it and the trophoblast in contact with it become membra- 

 naceous. They project into the uterine cavity and obhterate 

 its lumen by meeting, but not fusing with, the mesometrial 

 part of the uterine mucosa. As in the bat, the circulation in 

 the yolk-sac never ceases entirely during pregnancy. 



The changes in the allantoidean trophoblast are of the same 



Fia. 113. — The extension of the yolk-sac against the lacunar trophoblast in 

 Erinaceua (Hubrecht). The yolk-sac is to the left of the figure, and its 

 villi (Vi.) and blood-vessels (B.V.) are well seen. 



Tr,, trophoblast; Trs., trophospongia ; Df., layer of deciduofraots ; D., 

 decidua, of which the inner layer (£>') has assumed a. more reticulate 

 aspect ; Sp., spaces in trophoblast. 



kind, but they occur later. It occupies a discoid area as in 

 Rodents, but it is on the anti-mesometrial side, i.e. the primary 

 decidua reflexa is permanent. The lacunar spaces in it are more 

 comphcated than in the omphaloidean trophoblast, and their 

 walls bulge towards the embryo and interlock with vascular 

 projections of the allantois. Hence the viUi have a complete 

 trophoblastic covering. The extremity of each villus is attached 

 to the maternal decidua by strands of trophoblastic cells. The 

 allantoidean trophospongia develops Uke the omphaloidean, but 



