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 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, 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, ITT.). 
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. 
