The Development of the Alligator 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 11-11k (PLATES XVI., 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 (v), 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 (d) 
now covers nearly two thirds of the embryo. The 
heart (At) 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 
