450 THE PHYSIOLOGY OF REPRODUCTION 



With the changes in the mucosa, changes also take place in 

 the trophoblast. After the thinning already mentioned, its 

 cells increase in number. They grow in strands, leaving spaces 

 between them like the meshes of a net, and in the spaces 

 maternal blood circulates. In this respect the hedgehog differs 



Ily 





, 



Trs. 



FIG. 112. Section in situ of the ovum of Erinaceua (Hubrecht). 



Hy., hypoblast; Tr., trophoblast; sp., spaces in the trophoblast, communi- 

 cating with the maternal blood-spaces (M.Sp.) ; D., decidua ; Trs., 

 tropbospongia. 



from the Rodents, in which the proliferation of the trophoblast 

 is confined to the allantoic region. In the hedgehog the pro- 

 liferation 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 vitel- 

 line vessels, are developed. The omphaloidean placenta thus 



