l6 SMITHSONIAN MISCELM OLLECTIONS VOL. 5 1 



und in [905, and appr quite closely the condition of 



the chick embryo after 24 hours' incubation. The dorsal aspect of 

 this embryo, viewed by transmitted 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 (18) 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 

 medullar) folds arc ^till wid arated; ; rly 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 primi- 

 tive streak i-. 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 (5) figures the blastopore at this stage 

 ;i ;i small opening in fronl of the primitive streak, bul 'I 1 Men- 



tion any such condition as above described at any stage of devel 



it. Five pain of somites (s) hav< be< ted and may be 



ecu. though hut faintly outlined, in both dorsal and ventral views of 

 the embryo ; thi , lie aboul half way between the extreme end- of the 

 1 mhryo. The head-fold (7/, lig. 8a) shows plainly in a ventral view 

 as a darker, more opaque anterior region, extending for about one- 

 fourth the length of the embryo. The still unfused region of the 

 medullary fold- may be seen also in the ventral view at mg. The 

 I fold of the amnion (a) forms a very thin, transparent hood 

 r the extreme anterior end of the embryo. The tail-fold of the 

 amnion has not made its appearance, and in fact is not apparent at 

 any stage in the development. This is true also of the Madagascar 

 crocodile. The notochord (nt) ma; be seen , n a ventral view as a 

 faint, lim liny along the middle line from the head- 



told to the primitive streak. 



Two sagittal sections of thi hown in figures 8fc and 8c. 



The embryo from which the sections were made was apparently 

 somewhat crooked, so that it was not possible to get perfect longi- 

 tudinal sections. For example, in figure 8b the plan.- of the section 

 is almost exactly median in the extreme pO! terior and middle region--. 

 h"i i- on on.- id.- of the middle line elsewhere. Thi- explains the 

 enormou thickening of the ei toblast in the region of the head, wher< 



