266 The Alligator and Its Allies 
STAGE VII 
FIGuRES 10 AND 10a (PLATEs XV., XVI.) 
Although of practically the same size as the 
preceding, this stage has advanced sufficiently in 
development to warrant a description. 
The medullary folds are apparently still slightly 
open for the greater part of their length, though 
they are evidently fused together in the head 
region, except at theextremeend. Transverse sec- 
tions, however, of Figure 12, in which the medul- 
lary folds, from the dorsal aspect, seemed open 
(mg) as in Figure 10, have shown that these folds 
are fused throughout their length. 
The first cerebral vesicle (v’) is clearly indicated as 
an enlargement of the anterior end of the nervous 
system, and aslight enlargement (v”) posterior to the 
first probably represents the second cerebral vesicle. 
There are now eight pairs of somites (s). 
The head-fold (2) now shows in both dorsal and 
ventral views, appearing in the former, when 
viewed by transmitted light, as a lighter, circular 
area on either side of the body, just posterior to the 
hinder edge of the amnion. 
The head-fold of the amnion (a) has extended 
about twice as far backward as it did in the preced- 
ing stage. 
Owing to the opacity caused by the medullary 
folds being in contact along the middle line, the 
notochord is no longer visible in surface views. 
