The Development of the Alligator 237 



in this embryo was probably not so far developed 

 as it was in the embryo shown in Figures 3 and 3*7 . 

 Not having seen the embryo, however, before it was 

 sectioned, the writer cannot be certain of this 

 point. The ectoderm and entoderm are here of 

 nearly the same thickness. 



Figure 3^ is a short distance posterior to the 

 preceding. It shows a marked thickening of the 

 ectoderm in the medial region (cc), which is con- 

 tinuous posteriorly w r ith the anterior ends of the 

 medullary folds that are just beginning to differ- 

 entiate (Figs. 3/-/0- 



Figure 3g passes through the anterior end of the 

 medullary plate or folds (nif) , whichever they may 

 be called. The ectoderm of the folds is thickened 

 and is considerably elevated above the rest of the 

 blastoderm. There is scarcely any sign, in this 

 region, of a medullary groove. The entoderm (en) 

 is considerably thickened in the medial region, this 

 thickening being continuous posteriorly, as in the 

 preceding stage, with the mesoderm. 



In Figure 3^, cut in a plane at some distance 

 posterior to the preceding, the medullary groove 

 (mg) is well marked; its bordering folds grad- 

 ually thin out laterally to the thickness of the 

 ordinary ectoderm. The medial thickening of 

 the entoderm is very marked, but it has not in 

 this region separated into a distinct mesoblastic 

 layer. 



Immediately under the medullary groove is a 



