DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE II 



Stage III 

 Figures 4, 4a, 5, 5a, and 6-6i (Plates V, VI) 



"Figures 4 arf& 4a are of an embryo removed, on June 18, from an 

 egg which had been taken out of an alligator two days before. Fig- 

 ure 4, 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 point- 

 ing 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 figure 40, 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 medul- 

 lary 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 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. 5a) repre- 

 sents the thickened process on the mid-line at its greatest develop- 

 ment. 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 repre- 

 sented by figures 5 and 5a. 



"This is so unexpected a method of formation for the anterior 

 part of the medullary folds that I have made use of both figures 4 

 and 5. They were made from very perfect specimens, and the sec- 

 tions of both of them, and of the specimen from which figure 6 was 

 drawn, proves that the structure is what it is indicated to be in sur- 

 face appearance. That is, the transverse sections posterior to the V, 

 in the embryos shown in figures 4 and 5, show the medullary groove 

 and the medullary folds; the several sections passing through the 

 apex of the V show neither groove nor folds, but only a median 

 thickening ; and in front of the point or apex of the V the successive 

 sections discover a gradually widening groove between the arms, 

 which is also much deeper than the shallow groove found posterior 

 to the V. While I have not seen, and from the nature of the condi- 

 tions one cannot see, the change actually proceeding from the form 



