822 DEVELOPMENT. [CH. uvill. 



cavity, the allantois is thereby divided at the umbilicus into two 

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

 chorion, soon shrivels ; 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, extends from the bladder to the 

 umbilicus, under the name of the urachus. After birth the um- 

 bilical 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, imme- 

 diately beneath the parietal layer of the peritoneum. 



Fig. 639. Fig. 640. 



Figs. 63Q and 640. n, chorion with villi. The villi are shown to be best developed in the 

 part of the ohorion to which the allantois is extending ; this portion ultimately be- 

 comes the placenta ; b, space between the true and false 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. 



The Chorion. This is formed by the fusion of three 

 membranes, namely, the original vitelline membrane, the outer 

 layer of the amniotic fold (false amnion), and the allantois which 

 supplies it with blood-vessels. 



Very soon after its formation, its outer surface is beset with 

 fine processes, chorionic villi (a, figs. 639, 640), which give it a 

 rough and shaggy appearance. At first only cellular in structure, 

 these little outgrowths subsequently become vascular by the 

 development in them of loops of capillaries (fig. 641); and the 

 latter at length form the minute extremities of the blood-vessels 

 which are conducted from the foetus to the chorion by the allantois. 

 The function of the villi of the chorion is evidently the absorption 

 of nutrient matter for the foetus ; and this is probably supplied to 



