368 GENERATION. 



According to Robin, the umbilical vesicle has three coats ; 

 an external, smooth membrane, formed of connective tissue, 

 a middle layer of transparent, polyhedric cells, and an inter- 

 nal layer of spheroidal cells. The membrane, composed of 

 these layers, encloses a pulpy mass, composed of a liquid, 

 containing cells and yellowish granulations. 1 



Formation of the Allantois and the Permanent Chorion. 

 During the early stages of development of the umbilical 

 vesicle, and while it is being shut off from the intestine, there 

 appears an elevation at the posterior portion of the intestine, 

 which rapidly increases in extent, until it -forms 'a membrane 

 of two layers, which is situated between the internal and 

 the external layer of the amnion. This membrane becomes 

 vascular early in the progress of its development, increases 

 in size quite rapidly, and finally completely encloses the in- 

 ternal layer of the amnion and the embryon, the gelatinous 

 mass already described being situated between it and the in- 

 ternal amniotic layer before this membrane becomes enlarged. 

 While the formation of the two layers of the allantois is quite 

 distinct in certain of the lower orders of animals, in the hu- 

 man subject and in mammals, it is not so easily observed ; 

 still, as was first shown by Coste, 2 there can be no doubt 

 as to the mechanism of its formation, even in the human 

 ovum. Here, however, the allantois soon becomes a single 

 membrane, the two original layers of which cannot be sepa- 

 rated from each other. The process of the development of the 

 allantois is shown in the diagrammatic figures 34, 35, and 36. 



It is the vascularity of the allantois which causes the rapid 

 development by which it invades and finally supersedes the 

 external layer of the amnion, becoming the permanent cho- 

 rion, or external membrane of the ovum. At first there are 

 two arteries extending into this membrane from the lower 



1 ROBIN, Memoire sur la structure intime de la vesicule ombilicale. Journal de 

 la physiologic, Paris, 1861, tome iv., pp. 308, 314. 



2 COSTE, Embryogenie comparee, Paris, 1837, tome i., p. 231, et seg. 



