QO EARLY EMBRYOLOGY OF THE CHICK 



The Allantois. The allantois differs from the amnion and 

 serosa in that it arises primarily within the body of the embryo. 

 Its proximal portion is intra-embryonic throughout develop- 

 ment. Its distal portion, however, is carried outside the con- 

 fines of the intra-embryonic ccelom and becomes associated with 

 the other extra-embryonic membranes. Like the other extra- 

 embryonic membranes the distal portion of the allantois 

 functions only during the incubation period and is not incorpor- 

 ated into the structure of the adult body. 



The allantois first appears late in the third day of incubation. 

 It rises as a diverticulum from the ventral wall of the hind-gut 

 and its walls are, therefore, splanchnopleure. Its relationships 

 to structures within the embryo will be better understood when 

 chicks of three and four days incubation have been studied, but 

 its general location can be appreciated from the schematic 

 diagrams of Figures 32 and 33. 



During the fourth day of development the allantois pushes 

 out of the body of the embryo into the extra-embryonic ccelom. 

 Its proximal portion lies parallel to the yolk-stalk and just 

 caudal to it. When the distal portion of the allantois has 

 grown clear of the embryo it becomes enlarged (Fig. 32, C). 

 Its narrow proximal portion is known as the allantoic stalk, 

 the enlarged distal portion as the allantoic vesicle. Fluid 

 accumulating in the allantois distends it so the appearance of 

 its terminal portion in entire embryos is somewhat balloon-like 

 (Fig. 40). 



The allantoic vesicle enlarges very rapidly from the fourth 

 to the tenth day of incubation. Extending into the sero- 

 amniotic cavity it becomes flattened and finally encompasses 

 the embryo and the yolk-sac (Fig. 30, C, D). In this process 

 the mesodermic layer of the allantois becomes fused with the 

 adjacent mesodermic layer of the serosa. There is thus formed 

 a double layer of mesoderm, the serosal component of which is 

 somatic mesoderm and the allantoic component of which is 

 splanchnic mesoderm. In this double layer of mesoderm an 

 extremely rich vascular network develops which is connected 

 with the embryonic circulation by the allantoic arteries and 

 veins. It is through this circulation that the allantois carries 

 on its primary function of oxygenating the blood of the embryo 

 and relieving it of carbon dioxide. This is made possible by the 



