744 



AVES. 



becomes distinctly separate from the yelk and the membrana vitelli, 

 and, as it begins to spread, assumes the form of a central pellucid spot, 

 surrounded by a broad dark ring (fig. 371, g, h) ; it at the same time 

 becomes thickened and prominent, and is soon separable into three 

 layers ; of these, the exterior (fig. 372, c) is a serous layer ; the internal, 



Fig. 371. 



Egg at commencement of incubation. 



or that next the yelk (A), a mucous layer ; and between the two is 

 situated a vascular layer (B), in which vessels soon become apparent. 

 These three layers are of the utmost importance, as from the first- 

 mentioned all the serous structures, from the second all the mucous 

 structure, and from the third the entire vascular system of the embryo 



originate. 



Fig. 372. 



Earliest appearance of embryo. 



(2114.) Towards the close of the first day of incubation the blasto- 

 derm has already begun to change its appearance, and two white fila- 

 ments are apparent in the middle of the central pellucid circle. Sup- 

 posing a longitudinal section of it at this period, the membrana vitelli 



