The Development of the AHigator 267 



The head at this stage begins to push down into 

 the yolk in a strange way that will be described 

 later. 



STAGE VIII 



Figures ii-ii^ (Plates XVL, XVII., XVIII.) 



This stage is about one fourth longer than 

 the preceding. The medullary canal is enclosed 

 throughout its entire length, though it appears in 

 surface view (Fig. 11) to be open in the posterior 

 half {mc) of the embryo. An enlargement of this 

 apparently open region at the extreme posterior 

 end {pg) is probably caused by the remains of the 

 primitive groove or the neurenteric canal, and a 

 slight opacity at the same point may be caused 

 by the primitive streak. The anterior end of the 

 neural tube is bent in a ventral direction (z'), as in 

 the preceding stage. The somites {s) now number 

 fifteen pairs; they are somewhat irregular in size 

 and shape. 



The head-fold is not so striking a feature as in the 

 preceding stage. The head-fold of the amnion {a) 

 now covers nearly two thirds of the embryo. The 

 heart (///) is seen as a dark, rounded object pro- 

 jecting to the right side of the neural canal, just 

 anterior to the first somite. The vitelline blood- 

 vessels are just beginning to form, but are not 

 shown in the figure. 



The depression of the anterior region that was 



