TIIK POSTAL Ml 



I'M 



(1) Till' pennalil-like prol<>i,i.Mtio!i of the Mini. 



r.eiM-ath thi-. abundantly de\.-!op.-d inf 1 r\ Mnic connective 



(3) Tlir fundament of the allantois, which has the form <>f a very 

 narn\v passage with epithelial liiiini: : 



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

 upon the allantoic duct, while the V( ins run nearer to the amnion. 



To the 4iie.stion, H,,\v have tl,< arisen t that appears to me 



Fig. 142. Diagram of the foetal membranes of a Mammal, after TI 



-na pellucida with villi (prochorion) ; .-:, eeroiis uiciuhrane ; u.ii, aumion AC, amniotic 

 ; E, outer germ-layer ; M, inul-llc Kt'i'i-l'\viT ; //, inner genn-layev ; UV, yolk-sac 

 (vesica umbilicalis) ; al, allantoia; ALC, alia 



the most natural answer which permits of being harmonised \\ith 

 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 knot) composed 

 of many cells, and containing only a smill ex-agination of the ento- 

 dennic layer The allantoic knob does not , IIOW.-V.T. ^row free into 

 the body-cavity, as in the remaining .Mammals (tiu r . 1-1- ul). but ex- 

 tends alonj; the ventral wall of the embryo, and. from the }>lace where 

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



