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 3&-W are drawn from transsections of an 

 embryo of about this stage of development. For a 

 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 3Z) 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 T^d passes through the head-fold, which 



