782 HANDBOOK OF PHYSIOLOGY. 



outer membrane or false chorion, where, by interlacement with the vas- 

 cular 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 shrivelling ; 

 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 

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

 shrivelled 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, 



Fig. 477. 



Fig. 476. Diagram of fecundated egg. a, umbilical vesicle; 6, amniotic cavity ; c, allantois. 

 'J)alton.) 



Fig. 477. Fecundated egg with allantois nearly complete, a, inner layer of amniotic fold; 

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

 seen penetrating 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. 

 478, 479, in which will be found the structures external to these folds. (Dalton.) 



immediately beneath the parietal layer of the peritoneum. It is some- 

 times enumerated among the ligaments of the bladder. 



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

 cessively 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 in- 

 vesting 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 joined with the vitelline membrane. 

 By these means the external investing membrane of the ovum, or the 

 true chorion, as it is now called, represents three layers, namely, the 

 original vitelline membrane, 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 



