DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 9 



Figure 3d passes through the head-fold, which in this embryo was 

 probably not so far developed as it was in the embryo shown in fig- 

 ures 3 and 3a. Not having seen the embryo, however, before it was 

 sectioned, the water cannot be certain of this point. The ectoderm 

 and entoderm are here of nearly the same thickness. 



Figure 3c is a short distance posterior to the preceding. It shows 

 a marked thickening of the ectoderm in the medial region (cc), 

 which is continuous posteriorly with the anterior ends of the medul- 

 lary folds that are just beginning to differentiate (figs. $f-h). 



Figure 3^ passes through the anterior end of the medullary plate 

 or folds (mf), 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 medul- 

 lary 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 pre- 

 ceding, the medullary groove (mg) is well marked; its bordering 

 folds gradually 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 dense mass of cells 

 (nt), apparently the anterior end of the notochord in process of 

 formation. 



Figure 31, still farther toward the blastopore, shows the medullary 

 groove wider and shallower than in the more anterior sections. The 

 mesoderm (mes) is here a layer laterally distinct from the entoderm. 

 In the middle line it is still continuous with the entoderm, and at this 

 place it is the more dense mass of cells that may be recognized as 

 the notochord (nt). It is evidently difficult to decide whether this 

 group of cells (nt) . which will later become a distinct body, the noto- 

 chord, is derived directly from the entoderm or from the mesoderm, 

 which is itself a derivative of the entoderm. There is here abso- 

 lutely no line of demarcation between the cells of the notochord and 

 those of the mesoderm and entoderm. 



In figure 3/ the ectoderm (ec) is nearly flat, scarcely a sign of the 

 medullary groove appearing. The mesoderm (mes) is here a dis- 

 tinct layer, entirely separate from both notochord (nt) and entoderm 

 (en). The notochord is a clearly defined mass of cells, distinct, as 

 has been said, from the mesoderm, but still closely united with the 

 underlying entoderm, which is much thinner than the ectoderm. 



