DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 1 3 



(th) that has been mentioned by Clarke (see above). Springing 

 from the entoderm on each side of this thickening is a small mass of 

 mesoderm (mes). 



The section immediately posterior to the one just described is rep- 

 resented in figure 6c. The line (/) which separated the two second- 

 ary folds in the preceding section is no longer present, so that the 

 ectoderm (ec) is continuous from side to side, with only a shallow 

 depression (nig), which may be considered as the extreme anterior 

 end of the medullary groove. The median thickening (th) is cut 

 near its posterior limit and still shows a close fusion of the germ 

 layers. There is no line of demarcation between the gradually flat- 

 tening secondary folds of the anterior end of the embryo and the 

 just forming medullary folds of the posterior end, so that it is im- 

 possible to say whether the thickening 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 medullary thickening of the ectoderm (ec) is 

 still marked and the shallow medullary groove (trig) is fairly dis- 

 tinct. The entoderm (en) is medially continuous with both meso- 

 derm (mes) and notochord (nt), though these two tissues are other- 

 wise 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 em- 

 bryo. 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. 



Figure 6h passes through the blastopore (blp). The appearance 

 of the section is almost identical with that of figure 2.f, already de- 

 scribed. 



Figure 6i is five sections posterior to the preceding and has about 

 the same structure as the corresponding sections in the precedmg 

 two stages, where this region of the embryo was injured, and hence 

 not drawn. Continuous with the posterior border of the blastopore 

 (seen in the preceding figure) is the deep furrow, the primitive 

 groove (pg). The ectoblast (ec) bordering this groove is much 



