56 THE FIRST DAT. [CHAP. 



absolutely, but also relatively to the pellucid area, 

 longer than it was at the 12th hour. 



It finally occupies about two-thirds of the length of 

 the area peUucida; but its hinder end in many instances 

 appears to stop short of the posterior border of the 

 area peUucida (Fig. 16). The median line of the 



Fig. 16. 



S DEFACE View of the Area Pbllucida of a Chick's 

 Blastoderm shortly after the formation of the 

 Primitive Groove. 



Tpr. primitive streak with primitive groove ; 

 af. amiiiotio fold. 



The darker shading round the primitive streak shews the 

 extension of the mesoblast. 



primitive streak becomes marked by a shallow furrow 

 running along its axis. In fresh specimens, viewed with 

 transmitted light, this furrow appears as a linear trans- 

 parency, but in hardened specimens seen under reflected 

 light may be distinctly recognized as a narrow groove, 



