The Development of the Alligator 241 



ary fold taking place in the head-fold. This grows 

 posteriorly along the median dorsal line, forming 

 a V-shaped process with the apex pointing back- 

 ward toward the blastopore. There is quite a 

 deep groove between the arms of the V. The head- 

 fold on the ventral side, as seen in Figure 4, made 

 from the same embryo as Figure 4, grows most 

 rapidly on the mid-line, and also becomes thicker at 

 that place. The medullary folds now begin to 

 form on either side of the medullary groove, end- 

 ing posteriorly on either side of the blastopore and 

 anteriorly on either side of the point of the V- 

 shaped process in the middle of the head-fold. 

 This is seen in Figure 5, which is a dorsal view of an 

 embryo from an egg three days after it was taken 

 out of an alligator. A ventral view of the same 

 embryo (Fig. 50) represents the thickened process 

 on the mid-line at its greatest development. For 

 some reason the notochord did not show in this 

 embryo, possibly owing to particles of the yolk 

 material adhering about the mid-line. 



" In an embryo a day or two older, the V-shaped 

 fold of the head-fold is seen to have broken through 

 at the apex, and each of the arms thus separated 

 from one another unites with the medullary fold 

 of its respective side. This can be seen in Figure 

 6, which is a dorsal view of part of an embryo a day 

 or two older than the one represented by Figures 

 5 and 5^7. 



"This is so unexpected a method of formation 



16 



