238 The Alligator and Its Allies 



dense mass of cells {nt), apparently the anterior 

 end of the notochord in process of formation. 



Figure 3/, 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 (;//). It is evidently difficult to 

 decide whether this group of cells (vt), which will 

 later become a distinct body, the notochord, is 

 derived directly from the entoderm or from the 

 mesoderm, which is itself a derivative of the ento- 

 derm. There is here absolutely no line of demar- 

 cation between the cells of the notochord and those 

 of the mesoderm and entoderm. 



In Figure 37 the ectoderm {ec) is nearly flat, 

 scarcely a sign of the medullary groove appearing. 

 The mesoderm {?nes) is here a distinct layer, 

 entirely separate from both notochord (nt) and 

 entoderm (eft) . The notochord is a clearly defined 

 mass of cells, distinct, as has been said, from the 

 mesoderm, but still closely united with the under- 

 lying entoderm, which is much thinner than the 

 ectoderm. This condition of the notochord, 

 which is found throughout about one third of the 

 length of the embryo, would give the impression 

 that the notochord is of a distinctly entodermal 

 origin. 



