THE URACHUS. 



587 



(Fig. 222 s ) 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 in other 

 mammalia, the vessels carried out by the allantois are distrib- 

 uted 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 abdominal 

 cavity, the allantois is thereby divided at the umbilicus into 

 two portions ; the outer part, extending from the umbilicus to 

 the chorion (p. 588), soon shrivelling ; while the inner part, 

 remaining in the abdomen, is in part converted into the urin- 



FlG. 223. 



FIG. 224. 



a, chorion with villi. The villi are shown- to be best developed in the part of the 

 chorion to which the allantois is extending; this portion ultimately becomes the 

 placenta, b, space between the two layers of the amnion. c, amniotic cavity, d, 

 situation of the intestine, showing its connection with the umbilical vesicle, e, um- 

 bilical vesicle. /, situation of heart and vessels, g, allantois (after Todd and Bowman). 



ary 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 imper- 

 vious cord stretched from the top of the urinary bladder to the 

 umbilicus, in the middle line of the body, immediately be- 

 neath 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 successively 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. 



