56 



THE FIRST DAY. 



[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 pellucida; but its hinder end in many instances 

 appears to stop short of the posterior border of the 

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



Fm. 16. 



ScjuFACE View of the Area Pellucida of a Chick's 

 Blastoderm shortly after the formation of the 

 Primitive Groove. 



pr. primitive streak with primitive groove ; 

 af. amniotic 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, 



