THE PLACENTA. 543 



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 

 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 

 emphasize 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 

 the allantois and yolk-sac. It ought also to be mentioned 

 that, mainly by a folding of the uterine wall, the hedgehog 

 embryo is virtually enclosed in a maternal sheath, homolog- 

 ous with a fold called the decidua reflexa in human embry- 

 ology, and analogous with a similar capsule in the rabbit. 



To sum up: — 



(i) At an early stage, a wall of epiblast encloses an aggre- 

 gate of hypoblast. 



(2) The epiblast divides into an embryonic disc and an 



outer blastocyst wall, with fixing and nutritive func- 

 tions, — 'the trophoblast. 



(3) The hypoblast becomes a sac, of which the upper 



portion lines the gut, while the lower part forms the 

 yolk-sac. 



(4) The mesoblast divides into somatic and splanchnic 



layers; a double fold of the somatic layer (along with 

 a slight sheet of epiblast) forms the amnion, of which 

 the outer limbs unite as the subzonal membrane, and 

 form along with the external epiblast — the diplotro- 

 phoblast. The splanchnic layer of the mesoblast is 

 continued round the yolk-sac. 



(5) The allantois grows out from the hind region of the 



gut, being lined internally by hypoblast, externally by 

 splanchnic mesoblast. The allantois plus the diplo- 

 trophoblast always forms the true placenta. 



(6) Part of the yolk-sac wall, uniting with the diplotro- 



phoblast, also forms an efficient but temporary 

 placenta. 



