242 The Alligator and Its Allies 



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 

 sections 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 surface 

 appearance. That is, the transverse sections pos- 

 terior to the V, in the embryos shown in Figures 4 

 and 5, show the medullary groove and the medul- 

 lary 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 conditions one cannot 

 see, the change actually proceeding from the form 

 of Figure 5 to that of Figure 6, still the explanation 

 given appears to be the only one possible" (17). 



A somewhat extended series of transverse sec- 

 tions of an embryo of about this age is represented 

 in figures 6a-i. 



Figure 6a is a section through the head-fold; it 

 passes through the extreme anterior end of the 

 secondary folds (sf) that were described, in surface 

 view, above (Figs. 5 and 6). The section was not 

 quite at right angles to the long axis of the embryo, 

 so that the fold on the right was cut farther toward 

 its anterior end than was the fold on the left. 



