DEVELOPMENT AND PLACENTATION. 653 



The embryo lay at first in a groove of the uterine wall, 

 moored by the preliminary blastocyst villi, which are as it 

 were pathfinders for those subsequently developed from 

 yolk sac and allantoic regions. At the point of attachment, 

 the mucous lining of the uterus ceases to be glandular, and 

 becomes much more vascular. As the embryo becomes 

 fixed, the blastocyst almost eating its way in, the outer 

 epithelium degenerates and disappears ; below this the outer 

 layer of the mucous membrane becomes spongy and exhibits 

 unique blood spaces, forming what Hubrecht calls the tro- 

 phospongia; below this there is the vascular and vitally 

 active remainder of the mucosa, less modified than the 





FIG. 228. Diagram of Foetal Membranes. (After 

 TURNER.) 



, Embryo ; ff, gut lined by hypoblast dotted ; the dark is meso- 

 blast ; UV, umbilical vesicle or yolk sac; AC, amniotic cavity; 

 ant, amnion proper ; sz>, sub-zonal membrane ; AL,C, allantoic 

 cavity ; al., allantois ; J./., may be here taken to represent the early 

 epiblastic trophoblast. 



above mentioned sponge ; below this again, there are the 

 muscular and other elements of the uterine wall, with which 

 we are not now concerned. The most important fact to 

 emphasise is, that the maternal blood in the spaces of the 

 spongy outer layer of the mucous membrane directly bathes 

 the foetal tissue represented by the trophoblast. By the 

 activity of the trophoblast cells, the nutritive and respiratory 

 advantages of the maternal blood are secured for the villi of 



