AMNION AND ALLANTOIS. 739 



The umbilical vesicle is accordingly lined by a portion of the internal 

 blastodermic layei;, continuous with the mucous membrane of the intes- 

 tine ; and covered by a portion of the external blastodermic layer, con- 

 tinuous with the integument of the abdomen. 



After the young animal leaves the egg, the umbilical vesicle in some 

 species becomes shrunken and atrophied by the absorption of its con- 

 tents ; while in others, the abdominal walls gradually extend over it, 

 and crowd it back into the abdomen; the nutritious matter which it 

 contains passing from the cavity of the vesicle into that of the intes- 

 tine by the narrow passage remaining open between them. 



In the human species, on the other hand, as well as in quadrupeds, 

 the umbilical vesicle becomes more completely separated from the ab- 

 domen. There is at first a wide communication be- 

 tween the cavity of the umbilical vesicle and that of Fig. 254. 



the intestine ; subsequently this communication is 

 gradually narrowed by the constriction of the ab- 

 dominal walls; and this constriction proceeds so far 

 that the opposite surfaces of the canal at least come 

 in contact with each other and adhere together, so 

 that the passage previously existing, between the 

 cavitjr of the intestine and that of the umbilical 

 vesicle, is obliterated, and the vesicle is then con- 

 nected with the abdomen only by an impervious 

 cord. This cord afterward elongates, and becomes with umbilical vesi- 



converted into a slender pedicle (Fig. 254), emerging aou 



from the abdomen of the foetus, and connected by its 

 farther extremity with the umbilical vesicle, which is filled with a trans- 

 parent, colorless fluid. The umbilical vesicle is distinctly visible in the 

 human foetus so late as the end of the third month. After that period 

 it diminishes in size, and is gradually lost in the advancing development 

 of the neighboring parts. 



Amnion and Allantois. 



The amnion and allantois are two organs which can be best studied 

 in connection with each other, since they are closely related in physio- 

 logical importance; the office of the first being to pro vide for the forma- 

 tion of the second. The amnion is developed from the external blasto- 

 dermic layer ; the allantois from the internal Ia3^er. The amnion is so 

 called probably from the Greek d.tm?, a young lamb ; on account of its 

 having been first observed as a foetal envelope in this animal. The 

 name of the allantois is also derived from the Greek axxayr <mSij>$, owing 

 to its elongated or sausage-like form in the pig, and some other of the 

 domestic animals. 



In the frog's egg. the embryo is abundantly supplied with moisture, 

 air, and nourishment from without. The absorption of oxygen and of 

 albuminous liquids, and the exhalation of carbonic acid, so far as it 

 is produced, can readily take place through the simple membranes of 



