254 The Alligator and Its Allies 



Figure 8g passes through the middle part of the 

 head-fold, and shows that the medullary folds in 

 this region are fused below, but are widely sepa- 

 rated above, where their margins are markedly bent 

 away from the mid-line. Between the epidermal 

 and nervous layers of the ectoderm a considerable 

 mass of mesoderm cells (mes) is seen. The curious 

 appearance of the preceding four figures, as well 

 as the first three figures of the next stage, was at 

 first quite puzzling, until a model of the embryo 

 was made from a series of sections. It was then 

 plain, as would have been the case before, except 

 for the unusual depth dorso-ventrally of the head of 

 the embryo, why the medullary canal should at the 

 extreme anterior end be open both dorsally and 

 ventrally, while a few sections caudad it is open 

 only ventrally, and still farther toward the tail it is 

 again open both above and below. These condi- 

 tions are produced by the bending under of the 

 anterior region of the medullary folds, probably by 

 the formation of the head-fold. It is apparently 

 a process distinct from the ordinary cranial flexure, 

 which occurs later. The fusion of the medullary 

 folds to form a canal begins, as has been already 

 mentioned, near the anterior end, whence it extends 

 both forward and backward. Hence, if the ante- 

 rior ends of the medullary folds be bent downward 

 and backward, their unfused dorsal edges will come 

 to face ventrally instead of dorsally and sections 

 through the anterior part of this bent-under region 



