640 REPRODUCTION. 



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

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

 tents. -In others the abdominal walls gradually extend over it, and 

 crowd it back into the abdomen ; the nutritious matter which it con- 

 tains passing into the intestine by the narrow passage remaining 

 between them. 



In man, as well as in quadrupeds, the umbilical vesicle becomes more 

 completely separated from the abdomen. There 

 FIG. 193. is at first a wide communication between the two; 



but this communication is subsequently narrowed 

 by the gradual constriction of the abdominal walls, 

 and the opposite surfaces of the canal at last 

 come in contact and unite with each other. The 

 passage is thus obliterated ; and the umbilical 

 vesicle is then connected with the abdomen only 

 by an impervious cord. The cord afterward in- 

 creases in length, becoming a slender pedicle (Fig. 

 193), connected at its farther extremity with the 

 HUMAN EMBRYO, with um- umbilical vesicle, which is filled with a transparent, 



bilical vesicle ; about the . , 



fifth week. colorless fluid. The umbilical vesicle of the human 



foetus is distinctly visible until 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 closely related in their physiological 

 importance, since the first necessarily precedes the formation of the 

 second. The amnion is developed from the external layer of the 

 blastoderm ; the allantois from its internal layer. The amnion is so 

 called probably from the Greek a/mj, a young lamb; from its having 

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

 the allantois is derived from the Greek d^arfo?^, owing to its elon- 

 gated or sausage-like form in some of the domestic animals. 



Both these organs are connected with the nutrition of the embryo 

 within the egg. In birds and quadrupeds, the young animal, while 

 still enclosed by the membranes, reaches a high grade of organization ; 

 and the processes of absorption, respiration, and exhalation necessary 

 for its growth require a special organ for their accomplishment. This 

 organ, which brings the blood of the foetus into relation with external 

 sources of nutrition, is the allantois. 



In the frog and similar species, the internal blastodermic layer, form- 

 ing the lining membrane of the intestine, is everywhere inclosed by 

 the external layer, forming the integument. But in the higher animals 

 a portion of the internal layer, destined to produce the allantois, is 

 brought into contact with the external membrane of the egg for pur- 

 poses of exhalation and absorption ; and this can only be accomplished 



