The Development of the Alligator 251 
streak, but does not mention any such condition 
as above described at any stage of development. 
Five pairs of somites (s) have been formed and 
may be seen, though but faintly outlined, in both 
dorsal and ventral views of the embryo; they lie 
about half-way between the extreme ends of the 
embryo. The head-fold (A, Fig. 8a) shows plainly 
in a ventral view as a darker, more opaque anterior 
region, extending for about one fourth the length 
of the embryo. The still unfused region of the 
medullary folds may be seen also in the ventral 
view at mg. The head-fold of the amnion (a) 
forms a very thin, transparent hood over the 
extreme anterior end of the embryo. The tail 
fold of the amnion has not made its appearance, 
and in fact is not apparent at any stage in the 
development. This is true also of the Madagascar 
crocodile. The notochord (nt) may be seen in a 
ventral view as a faint, linear opacity extending 
along the middle line from the head-fold to the 
primitive streak. 
Two sagittal sections of this stage are shown in 
Figures 8b and 8c. The embryo from which the 
sections were made was apparently somewhat 
crooked, so that it was not possible to get perfect 
longitudinal sections. For example, in Figure 8b 
the plane of the section is almost exactly median 
in the extreme posterior and middle regions, but is 
on one side of the middle line elsewhere. This 
explains the enormous thickening of the ectoblast 
