4 8o THE PHYSIOLOGY OF REPRODUCTION 



proliferation of the trophoblast is confined to the allantoic region. 

 In the hedgehog the proliferation occurs even in the omphaloidean 

 region, which is vascularised by the area vasculosa. Here the 

 vacuolated trophoblast is gradually interlocked with vascular 

 processes of the mesoblast, and yolk-villi, containing branches of the 

 vitelline vessels, are developed. The omphaloidean placenta thus 

 formed embraces about one-half of the circumference of the 



n. v. ----- 



FIG. 141. Transverse section through the uterus of Sarex at a stage when 

 the blastocysts are still in the oviducts. The coiled uterine glands (<H.) 

 are massed together in the anti-mesometrial regions. The uterine lumen 

 ( U) is more or less _L-shaped. (From Hubrecht's " The Placentation of 

 the Shrew," Quar. Jour. Micr. Science, vol. xxxv., 1894.) 



B. \ r ., Blood-vessels ; c.m,., circular muscle ; l.m. t longitudinal muscle. 



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 membranaceous. 

 They project into the uterine cavity and obliterate 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 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 



