DEVELOPMENT AND FUNCTIONS OF THE ALLANTOIS. 665 



into the pleuro-peritoneal cavity. As it grows away from the 

 embryo it extends between the layers of the true and false amnion 

 and approaches towards the vitelline membrane, but remains con- 

 nected to the intestine by a narrow tube. When it reaches the 

 periphery of the ovum, it spreads over the chorion as a complete 

 lining, and sends processes into the villi of that organ. It becomes 

 chiefly developed, however, at that part of the chorion which is 

 opposite the decidua serotina of the mother. In the mesoblastic 



FIG. 256. 



Diagram of an embryo, showing the relationship of the vascular allantois 

 to the villi of the chorion. (Cadiat.) a, Lies in cavity of the amnion under 

 the embryo, b, Yolk-sac, c, Allantois. d, Vessels of the allantois dipping 

 into the villi of the chorion. e, Chorion. 



layer of the allantois bloodvessels arise which are connected with 

 large trunks, proceeding from the primitive aortse, called the um- 

 bilical arteries ; these will, however, be further described when 

 treating of the foetal placenta. 



As the foetus becomes developed, the part of the allantois in 

 connection with the body becomes gradually obliterated. A part 

 of it remains as the urinary bladder, and the rest forms a fibrous 



56 



