No. 2.] EMBRYOLOGY OF THE AMERICAN ALLIGATOR. 193 



which is a ventral view a few hours later. 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 develop- 

 ment. The lighter curve in front of the head-fold is the begin- 

 ning 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. 15) shows that the medullary 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. 



Figs. 16 and 17 are of an embryo removed, on June 18, 

 from an egg which had been taken out of an alligator two days 

 before. Fig. 16, a dorsal view, is of special interest in that 

 it shows a secondary fold taking place in the head-fold. This 

 grows posteriorly along the median dorsal line, forming a 

 V-shaped process with the apex pointing backward 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 Fig. 17, 

 made from the same embryo as Fig. 16, grows most rapidly on 

 the mid-line, and also becomes thicker at that place. The me- 

 dullary folds now begin to form on either side of the medullary 

 groove, ending 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 Fig. 18, 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. 19) 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 Fig. 

 20, which is a dorsal view of part of an embryo a day or two 

 older than the one represented by Figs. 18 and 19. 



This is so unexpected a method of formation for the anterior 

 part of the medullary folds, that I have made use of both Figs. 



