The Development of the Alligator 251 



streak, but does not mention any such condition 

 as above described at any stage of development. 

 Five pairs of somites (s) have been formed and 

 may be seen, though but faintly outlined, in both 

 dorsal and ventral views of the embryo; they lie 

 about half-way between the extreme ends of the 

 embryo. The head-fold (/?, Fig. 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 folds may be seen also in the ventral 

 view at mg. The head-fold of the amnion (a) 

 forms a very thin, transparent hood over 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 (;//) may be seen in a 

 ventral view as a faint, linear opacity extending 

 along the middle line from the head-fold to the 

 primitive streak. 



Two sagittal sections of this stage are shown in 

 Figures Sb and 8c. The embryo from which the 

 sections were made was apparently somewhat 

 crooked, so that it was not possible to get perfect 

 longitudinal sections. For example, in Figure 8b 

 the plane of the section is almost exactly median 

 in the extreme posterior and middle regions, but is 

 on one side of the middle line elsewhere. This 

 explains the enormous thickening of the ectoblast 



