472 



DEVELOPMENT OF THE CHICK. 



length of the primitive streak and terminates in front in 

 a small pit. 



Fig. 82. A diagrammatic figure of the blastoderm of a Hen"s Egg 

 about the twentieth hour of incubation. (In part after Duval.) 

 x 8. 



AD, area pellucida : the part left white consists of epiblast and hypoblast 

 alone : in the hinder part of the pyriform area, covered by the light shading, 

 mesoblast is present as well. AK, area opaca. M, dotted line indicating the 

 boundary of the mesoblast. NR, neural plate, the first commencement of the 

 central nervous system. PS, primitive streak. 



Transverse sections through the primitive streak show 

 that it is a keel-like band of proliferating epiblast (fig. 83), 

 and that the lower-layer or hypoblast cells have little or no 



Fig. 83. Transverse section across the blastoderm of a Hen's Egg about 

 the twentieth hour of incubation, the section passing through the 

 primitive streak about the middle of its length, x 200. 



E, epiblast. H, hypoblast. M, mesoblast. PG, primitive groove. PS, 

 primitive streak. 



share in its formation. The cells that it buds off migrate 

 into the spaces between the epiblast and hypoblast as a 

 paired sheet of loose tissue. This tissue is the mesoblast, 



