654 MAMMALIA. 



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 

 embryology, and analogous with a similar capsule in the 

 rabbit. 



To sum up : 



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



gate of hypoblast (Figs. 224, 225, I., 226). 



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



outer blastocyst wall, with fixing and nutritive func- 

 tions, the trophoblast (Figs. 225, 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. 225, 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 diplotro- 

 phoblast. The splanchnic layer of the mesoblast is 

 continued round the yolk sac (Fig. 227). 



(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 (Fig. 

 228). 



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



phoblast, 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 

 epiblasttc outer wall the trophoblast with its pre- 

 liminary path-finding villi. 



