250 The Alligator and Its Allies 
theegg. Thelength of the embryo proper is 3 mm. 
This was the youngest stage found in 1905, and 
approximates quite closely the condition of the 
chick embryo after 24 hours’ incubation. The 
dorsal aspect of this embryo, viewed by trans- 
mitted light, is shown in Figure 8. The medullary 
folds (mf) have bent over until they are in contact, 
though apparently not fused for a short distance 
near their anterior ends. As will be described 
in connection with the sections of this stage, the 
medullary folds are actually fused for a short 
distance at this time, though in surface views 
they appear to be separated from each other. In 
the Madagascar crocodile (78) the first point of 
fusion of the medullary folds is in the middle 
region of the embryo, or perhaps even nearer the 
posterior than the anterior end of the medullary 
groove. Throughout the greater part of their 
length the medullary folds are still widely sepa- 
rated; posteriorly they are merged with the sides 
of the very distinct primitive streak (ps), which 
seems, Owing to its opacity, to extend as a sharp 
point toward the head. Extending for the greater 
part of the length of the primitive streak is the 
primitive groove (pg), which, when the embryo 
is viewed by transmitted light, is a very striking 
feature at this stage of development and resembles, 
in a marked way, the same structure in the embryo 
chick. Clarke (17) figures the blastopore at this 
stage as a small opening in front of the primitive 
