672 HANDBOOK OF PHYSIOLOGY. 



with the external investing membrane of the egg. As it grows, the 

 allantois develops muscular tissue in its external wall and becomes ex- 

 ceedingly vascular; in birds (Fig. 457) it envelops the whole embryo- 

 taking up vessels, so to speak, to the outer investing membrane of the 

 egg, and lining the inner surface of the shell with a vascular membrane, 

 by these means affording an extensive surface in which the blood may be 

 aerated. In the human subject and other Mammalia, the vessels carried 

 out by the allantois are distributed only to a special part of the outer 

 membrane or false 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 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 



FIG. 456. Fia. 457. 



FIG. 456. Diagram of fecundated egg. a, umbilical vesicle; 6, amniotic cavity; c, allantois. 

 (Dalton.) 



FIG. 457. Fecundated egg with allantois nearly complete, a, inner layer of amniotic fold; &, 

 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. 453, 459, in which 

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



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, 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 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 C.horion. It has been already remarked that the allantois is a 

 structure which extends from the body of the foetus to the outer investing 

 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 



