X.] PRIMITIVE STREAK. 319 



todermic vesicle is therefore formed of three areas, 

 (1) the embryonic area with two layers, a columnar 

 epiblast and flat hypoblast; (2) the region around the 

 embryonic area where the walls of the vesicle are formed 

 of flattened epiblast 1 and of hypoblast ; (3) the area 

 beyond this again where the vesicle is formed of flat- 

 tened epiblast 1 only. 



The changes which next take place begin with the 

 formation of a primitive streak, homologous with, and in 

 most respects similar to, the primitive streak in Birds. 



FIG. 100. 



EMBRYONIC AREA OF AN EIGHT DAYS' RABBIT. 



(After Kolliker.) 

 arg. embryonic area ; pr. primitive streak. 



The formation of the streak is preceded by that of a 

 dark spot near the middle of the blastoderm, forming 

 the nodal point of Hensen. This spot subsequently 

 constitutes the front end of the primitive streak. 



Early on the seventh day the embryonic area be- 

 comes pyriform, and at its posterior and narrower end 



1 The epiblast of the blastodermic vesicle beyond the embryonic 

 area is formed of the outer layer only. 



