70S MAMMALIA. 



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 re- 

 spiratory 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, 

 homologous with a fold called the decidua reflexa in human 

 embryology, 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. 344, 345, I., 347). 



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



cyst wall, with fixing and nutritive functions, the trophoblast 

 (Fig. 345, 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. 345, 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. 348). 



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. 349). 



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



forms an efficient but temporary placenta. 



7. At the area of fixing, the uterine epithelium degenerates, the 



glands disappear, vascularity increases. The outer part of the 

 modified mucous membrane (or decidua) becomes a spongy 

 tissue, with spaces filled with maternal blood. This maternal 

 blood bathes the trophoblast, which is intermediate between it 

 and the placental villi. 



The three modes of embryonic nutrition are as follows : 



(a) At first the maternal blood bathes the lacunae in the epiblastic 

 outer wall the trophoblast with its preliminary pathfinding 

 villi. 



(/>) An efficient yolk-sac placenta functions for a time, but decreases 

 and shrivels as the final allantoidean placenta develops. The 



