FCETAL MEMBRANES 825 



exhibits unique blood spaces, forming what Hubrecht calk 

 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 spongv 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, 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 ectoderm encloses an aggregate of 



cells largely endodermal (Fig. 491, i.e.). 



2. The ectoderm becomes divided into an embryonic disc and an outer 



blastocyst wall, with fixing and nutritive functions — the tropho- 

 blast (Fig. 492, I. and II.). 



3. The endod;rm becomes a sac, of which the upper portion lines 



the gut, while the lower part forms the yolk-sac (Fig. 492, III.). 



4. The mesoderm divides into somatic and splanchnic layers ; a 



double fold of the somatic layer (along with a slight sheet of 

 ectoderm) forms the amnion, of which the outer limbs unite as 

 the subzonal membrane, and form, along with the trophoblast, 

 the diplotrophoblast. The splanchnic layer oi the mesoderm 

 is continued roimd the yolk-sac (Fig. 495). 



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



lined internally by endodcrm, externally by splanchnic meso- 

 derm. The allantois plus the diplotrophoblast always forms the 

 embryonic part of th > final placenta (Figs. 495, 496^-. 



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 vilU. 



