236 The Alligator and Its Allies 
“The head-fold rapidly increases in depth 
and prominence, as shown in Figure 3, which is a 
ventral view a few hours later [than the preceding 
stage]. The time cannot be given exactly, as it is 
found that eggs of the same nest are not equally 
advanced when laid, and differ in their rate of 
development. The lighter curve in front of the 
head-fold is the beginning of the anterior fold of the 
amnion. The notochord has been rapidly forming, 
and now shows very distinctly on the ventral side, 
when viewed by transmitted light. A dorsal view 
of the same embryo (Fig. 3a) shows that the medul- 
lary or neural groove is appearing, and that it ends 
abruptly anteriorly near the large transverse head- 
fold. Posteriorly it terminates at the thickened 
area in front of the blastopore, which still remains 
open.” 
Figures 3b-m are drawn from transsections of an 
embryo of about this stage of development. Fora 
short distance in front of the beginning of the head- 
fold, there is a mass of cells of considerable thick- 
ness between the ectoderm and entoderm. In 
Figure 30 these cells appear as an irregular thicken- 
ing of the entoderm, while in Figure 3c they form 
a continuous mass, uniting the upper and lower 
germ layers. This condition is seen, though in a 
much less striking degree, in the following stage 
of development. As to its significance the writer is 
not prepared to decide. 
Figure 3d passes through the head-fold, which 
