732 MAMMALIA. 



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 next 

 layer of the mucous membrane becomes spongy and exhibits 

 unique blood spaces, forming what Hubrecht calls the 

 trophospongia ; 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 

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

 spongy outer layer of the mucous membrane directly bathes 

 the fcetal 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, homo- 

 logous with a fold called the decidua reflexa in human em- 

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

 To sum up 



1. At an early stage a wall of epiblast encloses an aggregate of 



hypoblast (Figs. 395, 396, I., 398). ^ 



2. The epiblast divides into an embryonic disc and an'outer blasto- 



cyst wall, with fixing and nutritive functions, the trophoblast 

 (Fig. 396, I. and II.). 



3. The hypoblast becomes a sac, of which the upper portion lines 



the gut, while the lower part forms the yolk-sac (Fig. 396, III.). 



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 diplotrophoblast. The splanchnic layer of the 

 mesoblast is continued round the yolk-sac (Fig. 399). 



5 The allantois grows out from the hind region of the gut, being 

 lined internally by hypoblast, externally by splanchnic meso- 

 blast. The allantois plus the diplotrophoblast always forms the 

 true placenta (Fig. 400). 



6. Part of the yolk-sac wall, uniting with the diplotrophoblast, also 

 forms an efficient but temporary placenta. 



