MAMMALIA. 



221 



the embryonic area with three layers: this area is placed where 

 the blastopore was originally situated. (2) The ring around the 

 embryonic area where the walls of the vesicle are formed of epi- 

 blast and hypoblast. (3) The area beyond this again where the 

 vesicle is formed of epiblast only 1 . 



A. 



B. 



FlG. 137. VIEWS OF THE BLASTODERMIC VESICLE OF A RABBIT ON THE SEVENTH 



DAY WITHOUT THE ZONA. A. from above, B. from the side. (From Ko'lliker.) 

 ag. embryonic area ; ge. boundary of the hypoblast. 



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. The formation of the 

 streak is preceded by that of a clear spot near the middle of the 

 blastoderm, forming the nodal point of Hensen. This spot sub- 

 sequently constitutes the front end of the primitive streak. 



The history of the primitive streak was first worked out in a 

 satisfactory manner by Hensen (No. 182), from whom however I 

 differ in admitting the existence of a certain part of the meso- 

 blast before its appearance. 



Early on the seventh day the embryonic area becomes pyri- 

 form, and at its posterior and narrower end a primitive streak 

 makes its appearance, which is due to a proliferation of rounded 

 cells from the epiblast. At the time when this proliferation 



1 Schafer describes the blastodermic vesicle of the cat as being throughout in a 

 bilaminar condition before the formation of a definite primitive streak or of the 

 mesoblast. 



