258 The Alligator and Its Allies 



Figures 8 and 10. The amnion and head-fold are 

 nearly the same as in Figure 8 ; the medullary folds 

 are intermediate in development, the anterior end 

 not showing so marked an enlargement as shown in 

 Figure 10, v' . There are six or seven faintly dis- 

 tinguishable somites. 



Figure 9a represents a section through the 

 anterior part of the head-fold ; it shows one unusual 

 condition : the head lies entirely beneath the surface 

 of the yolk. This condition is quite confusing when 

 the section is studied for the first time. The push- 

 ing of the head under the yolk is shown at its com- 

 mencement in Figure 11. The process continues 

 until nearly the entire anterior half of the embryo 

 is covered ; but when the embryo attains a consider- 

 able size it is seen to lie entirely above the yolk, as 

 in the chick. According to Voeltzkow's figures 

 (78) this same condition is found in the crocodile, 

 and Balfour also mentions it in connection with 

 the lizard. The fusion of the medullary folds has 

 made considerable progress, so that the entire an- 

 terior end of the canal is enclosed, except in the 

 region where the folds are bent down and back, as 

 in the preceding stage; here the folds are still 

 distinct from each other, leaving the medullary 

 canal open on the ventral side, as shown in Figures 

 9 and 9&. In the section under discussion the 

 ectoderm {ec) is a very thin membrane on top of 

 a considerable mass of yolk, while no entoderm can 

 be distinguished. The amnion {(i) completely sur- 



