THE PRIMITIVE STREAK OF THE RABBIT. 197 



narrow dark line instead of a wide lighter line when seen 

 under similar conditions at the earlier stage. 



In all the stages described hitherto there has been no fusion 

 of the hypoblast with the proliferating epiblast. In stage fig. 

 5 at the extreme anterior end of this area, where there is a 

 special thickening of the primitive streak, the hypoblast is in 

 very close contact (see figs. 20 and 21). 



In this stage (fig. 6) the fusion is complete, and anteriorly 

 the hypoblast in the middle line and the epiblast and mesoblast 

 are continuous in the mass spoken of as Hensen's node. 



Figs. 24,, 25, 26, and 27 are sections taken along the lines 

 24, 25, 26, and 27 in fig. 6. 



A section taken more anterior still than fig. 24 corresponds 

 to figs. 13 and 19. 



The last three sections (figs. 25, 26, and 27) should be com- 

 pared with figs. 21, 22, and 23, and figs. 14, 15, and 16. Here 

 again it will be noticed that there is no great increase or 

 diminution of extent in transverse section of the area of 

 active proliferation. The most posterior parts differ hardly 

 at all except in the spreading of the separated cells (meso- 

 blast). 



It must be noticed that there is now a region hitherto not 

 to be found. The section drawn (fig. 24) passes through this 

 region, which is just anterior to the primitive streak. 



The section through the middle region (fig. 26; compared 

 with figs. 22 and 15) differs in one particular only (besides ex- 

 tension of mesoblast). Instead of the area of proliferation 

 appearing at the surface on a level with the rest of the outer 

 layer, it lies at the bottom of a deep groove. The sides of the 

 groove are not " fused " with mesoblast ; it is only the floor of 

 the groove that is participating in the addition to the meso- 

 blast. 



Fig. 25, the section through the anterior region of this area 

 of proliferation, shows an intensified form of the state in the 

 preceding stage (fig. 21). Here the groove suddenly ceases, 

 and even in most instances gives rise to an eminence. This 

 eminence so overhangs the groove posteriorly as to give rise to 



