336 The Alligator and Its Allies 
The medullary folds have a curious origin, 
difficult to explain without the use of figures. 
They are continuous posteriorly with the primitive 
streak, so that it is impossible to tell where the 
medullary groove ends and the primitive groove 
begins, unless the dorsal opening of the blastopore 
be taken as the dividing point. 
The amnion develops rapidly, and entirely from 
the anterior end. 
The blastopore or neurenteric canal is a very 
distinct feature of all the earlier stages up to about 
the time of closure of the medullary canal. 
Preceding the ordinary cranial flexure there is a 
sort of temporary bending of the head region, due 
apparently to the formation of the head-fold. 
During the earlier stages of development the 
anterior end of the embryo is pushed under the 
surface of the blastoderm, and is hence not seen 
from above. 
Body torsion is not so definite in direction as in 
the chick, some embryos lying on the right side, 
others on the left. 
Of the gill clefts, three clearly open to the exte- 
rior and probably a fourth also. A probable fifth 
cleft was seen in sections and in one surface 
view. 
The first trace of the urinary system is seen as a 
dorsally projecting, solid ridge of mesoblast in the 
middle region of the embryo, which ridge soon 
becomes hollowed out to form the Wolffian duct. 
