The Development of the Alligator 245 



secondary folds of the anterior end of the embryo 

 and the just forming medullary folds of the pos- 

 terior end, so that it is impossible to say whether 

 the thickenir'" of ectoblast in this figure should be 

 called secondary folds or medullary folds. As a 

 matter of fact, the secondary folds become, of 

 course, the anterior ends of the medullary folds. 

 The mesoblast (mes) is here of considerable extent, 

 and its entodermal origin is beyond doubt, though 

 not well shown in the figure. 



Figure 6f is about one sixth of the length of the 

 embryo posterior to the preceding. The medul- 

 lary thickening of the ectoderm (ec)is still marked 

 and the shallow medullary groove (mg) is fairly dis- 

 tinct. The entoderm (en) is medially continuous 

 with both mesoderm (mes) and notochord (;//), 

 though these two tissues are otherwise distinct 

 from each other. 



Figure 6g is nearly one third the length of the 

 embryo posterior to the preceding and passes 

 through the posterior third of the embryo. The 

 medullary thickening of the ectoderm (ec) is 

 marked, but shows no sign of a medullary groove; 

 in fact, the median line is the most elevated region 

 of the ectoderm. The notochord (nt) is larger 

 in cross-section than in the more anterior regions. 

 It is still continuous with the entoderm (en) and 

 is fairly closely attached to, though apparently 

 not continuous with, the mesoderm (mes) on each 

 side. 



