418 



THE PLACENTA AND GENERATIVE 



entirely disappear as a distinct sac, as in man. Between these extremes there are 

 a number of intermediate stages exhibited in mammals. Remembering, then, that 

 the permanency of the allantois as a distinct sac is very variable, but admitting 

 that at some period of gestation it is present in the elephant, it is incumbent upon 

 me to endeavor to explain what became of it in my specimen. As is well known, 



the allantois is a diverticulum of the posterior 

 part of the alimentary canal, and as this canal 

 is developed out of the hypoblast and part of 

 the mesoblast, necessarily the allantois will con- 

 sist of the same two layers, or darmdrusenblatt 

 and darmfaserblatt of the German embryologists. 

 The allantois expands into the amnio-chorionic 

 space, and gradually reaches the chorion. Its 

 outer layer, the mesoblastic part, the darmfaser- 

 blatt, or the vascular or exochorionic portion, 

 replaces or fuses with the chorion, and is the 

 layer in which the foetal vessels are found. It 

 is this layer which forms the foetal portion of 

 the placenta in my specimen. The inner layer 

 of the allantoic sac, which is a continuation of 

 the epithelial layer of the alimentary canal, the 

 hypoblastic part, the darmdrusenblatt, the mu- 

 cous or endochorionic layer, is, however, non- 

 vascular. Now suppose that the outer part of 

 this layer adheres to the vascular layer as the 

 vascular layer adheres to the chorion, and the 

 three membranes fuse their identity into one. 

 All trace, then, of the m^ucous unvascular endo- 

 chorionic part of the allantois will have disap- 

 a. Chorion, large poucii. a'. Chorion, peared as a distinct membrane, as is the case 

 small pouch, b, b'. Amnion, d. Umbilical .^^ ^^^ human being. On the other hand, if the 



cord. c. Placenta. i i i 



inner part of the mucous and vascular layers, 

 which lie against the amnion, should fuse with it, then that part of the mucous 

 layer of the allantois would also disappear in the course of development. Such a 

 mode of disappearance of the allantois seems to have taken place in the develop- 

 ment of the membrane of my specimen. For, on carefully examining the amnion, 

 I find that in a greater part of its extent it can be separated into two layers, the 



