264 HAND-BOOK OF PHYSIOLOGY. 



human subject and in other Mammalia, the vessels carried out by the 

 allantois are distributed only to a special part of the outer membrane or 

 chorion, where, by interlacement with the vascular system of the mother, 

 a structure called the placenta is developed. 



In Mammalia, as the visceral laminae close in the abdominal cavity, 

 the allantois is thereby divided at the umbilicus into two portions; the 

 outer part, extending from the umbilicus to the chorion, soon shriveling; 

 while the inner part, remaining in the abdomen, is in part converted 

 into the urinary bladder; the portion of the inner part not so converted, 

 extending from the bladder to the umbilicus, under the name of the 



FIG. 420. FIG. 421. 



Fro. 420. Diagram of fecundated egg. a, umbilical vesicle; 6, amniotic cavity; c, allantois. 

 (Dalton.) 



FIG. 421. Fecundated egg with allantois nearly complete, a, inner layer of amniotic fold; 6, 

 outer layer of ditto; c, point where the amniotic folds come in contact. The allantois is seen pene- 

 trating between the outer and inner layers of the amniotic folds. This figure, which represents only 

 the amniotic folds and the parts within them, should be compared with Figs. 417, 423, in which will be 

 found the structures external to these folds. (Dalton.) 



uraclms. After birth the umbilical cord, and with it the external and 

 shriveled portion of the allantois, are cast off at the umbilicus, while the 

 urachus remains as an impervious cord stretched from the top of the 

 urinary bladder to the umbilicus, in the middle line of the body, imme- 

 diately beneath the parietal layer of the peritoneum. It is sometimes 

 enumerated among the ligaments of the bladder. 



It must not be supposed that the phenomena which have been succes- 

 sively described, occur in any regular order one after another. On the 

 contrary, the development of one part is going on side by side with that 

 of another. 



The Chorion. It has been already remarked that the allantois is a 

 structure which extends from the body of the foetus to the outer invest- 

 ing membrane of the ovum, that it insinuates itself between the two layers 

 of the amniotic fold, and becomes fused with the outer layer, which has 

 itself become previously fused with the vitelline membrane. By these 

 means the external investing membrane of the ovum, or the chorion, as it 

 is now called, represents three layers, namely, the original vitelline mem- 

 brane, the outer layer of the amniotic fold, and the allantois. 



Very soon after the entrance of the ovum into the uterus, in the 

 human subject, the outer surface of the chorion is found beset with fine 



