250 The Alligator and Its Allies 



the egg. The length of the embryo proper is 3 mm. 

 This was the youngest stage found in 1905, and 

 approximates quite closely the condition of the 

 chick embryo after 24 hours' incubation. The 

 dorsal aspect of this embryo, viewed by trans- 

 mitted light, is shown in Figure 8. The medullary 

 folds (mf) have bent over until they are in contact, 

 though apparently not fused for a short distance 

 near their anterior ends. As will be described 

 in connection with the sections of this stage, the 

 medullary folds are actually fused for a short 

 distance at this time, though in surface views 

 they appear to be separated from each other. In 

 the Madagascar crocodile (78) the first point of 

 fusion of the medullary folds is in the middle 

 region of the embryo, or perhaps even nearer the 

 posterior than the anterior end of the medullary 

 groove. Throughout the greater part of their 

 length the medullary folds are still widely sepa- 

 rated; posteriorly they are merged with the sides 

 of the very distinct primitive streak (ps), which 

 seems, owing to its opacity, to extend as a sharp 

 point toward the head. Extending for the greater 

 part of the length of the primitive streak is the 

 primitive groove (pg), which, when the embryo 

 is viewed by transmitted light, is a very striking 

 feature at this stage of development and resembles, 

 in a marked way, the same structure in the embryo 

 chick. Clarke (17) figures the blastopore at this 

 stage as a small opening in front of the primitive 



