750 GENERATION AND DEVELOPMENT. 



union with the outer of the two folds, which has itself, as 

 before said, become one with the external investing mem- 

 brane of the egg. As it grows, the 

 allantois becomes exceedingly vascu- 

 lar, and in birds (fig. 215) envelopes 

 the whole embryo taking out vessels, 

 so to speak, to the outer investing 

 membrane of the egg, and lining the 

 inner surface of the shell with a vas- 

 cular membrane ; by these means afford- 

 ing an extensive surface in which the 

 blood may be aerated. In the human subject and in 

 other mammalia, the vessels carried out by the allantois 

 are distributed only to a special part of the outer membrane, 

 at which a structure called the placenta is developed. 



In Mammalia, as the visceral laminae close in the abdo- 

 minal cavity, the allantois is thereby divided at the umbi- 

 licus 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, immediately beneath the parietal layer of the 

 peritoneum. It is sometimes enumerated among the liga- 

 ments of the bladder. 



* Fig. 215. Fecundated egg with allantois nearly complete, a, 

 inner layer of amniotic fold ; b, outer layer of ditto ; , 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. 216, 217, in which will be found the 

 structures external to these folds. 



