THE F(ETAL MEMBRANES OF MAN. 



247 



(1) The pennant-like prolongation of the amnion ; 



(2) Beneath this, abundantly developed embryonic connective 

 tissue ; 



(3) The fundament of the allantois, which has the form of a very 

 narrow passage with epithelial lining ; 



(4) The umbilical blood-vessels, of which the arteries lie close 

 upon the allantoic duct, while the veins run nearer to the amnion. 



To the question, How have these parts arisen ? that appears to me 



fig. 142. Diagram of the foetal membranes of a Mammal, after TURNER. 



pc, Zona pellucida with villi (prochorion) ; sz, serous membrane ; am, amnion AC, amniotic 



cavity ; E, outer germ-layer ; M, middle germ-layer ; H, inner germ-layer ; DV, yolk-sac 



(vesica umbilicalis) ; al, allantois; ALC, allantoic cavity. 



the most natural answer which permits of being harmonised with 

 the known conditions in other Mammals. Now, such an agreement 

 is possible upon the following assumption. 



Very early, when the hind gut begins to be formed, there arises 

 on its ventral side as a fundament of the allantois a knob composed 

 of many cells, and containing only a small evagination of the ento- 

 dermic layer. The allantoic knob does not, however, grow free into 

 the body-cavity, as in the remaining Mammals (tig. 142 al), but ex- 

 tends along the ventral wall of the embryo, and, from the place where 

 this is reflected off to form the amnion, along the ventral wall of the 



