128 THE STRUCTURE OF THE FOWL 



coelome and quickly extends round the hinder 

 end of the embryo as a pear-shaped bladder. 

 It then rapidly enlarges and forms a flattened 

 sac between the chorion and amnion, with both 

 of which it enters into intimate union. From 

 the beginning the allantois is very vascular, 

 and consequently acts as an organ of respira- 

 tion and for the absorption of albumen. At 

 the same time it receives the excretion produced 

 by the embryonic kidneys. 



Seeing that the allantois is derived from 

 the splanchnopleure, it follows that it will 

 consist of mesoblast with an inner lining of 

 entoderm. 



Though the yolk-sac may be included 

 among the embryonic membranes, it differs 

 materially in origin and fate from the chorion, 

 amnion, and allantois. It commences as the 

 splanchnopleure surrounding the mass of the 

 yolk, but as development proceeds it neces- 

 sarily becomes gradually smaller consequent 

 upon the absorption of the yolk. At first it 

 is a simple rounded bag, but the folding-in of 

 the body-wall results in the pinching off of a 

 portion of the original sac and the inclusion 

 of this part within the embryo, where it forms 

 the rudiment of the alimentary canal. By 



