The Development of the Alligator 243 



The pushing under of the head causes a forward 

 projection of the secondary folds, so that the fold 

 to the right appears as a rounded mass of cells with 

 a small cavity near its center. On the left the 

 plane of the section passes through the posterior 

 limit of the head-fold, and shows the cells of the 

 secondary fold continuous with the dorsal side of 

 the ectoderm (ec) . As pointed out above by Clarke, 

 the secondary folds are here some distance apart, 

 and gradually approach each other as we proceed 

 toward the posterior. The entoderm (en) is here 

 flat and takes no part in the secondary folds. 



In Figure 6b, sl short distance back of the one 

 just described, the secondary folds (sf) are much 

 larger and are closer together. On the right the 

 section passes through the extreme limit of the 

 head-fold, so that the secondary fold of that side 

 is still a closed circle, with a few scattered cells 

 enclosed. On the left the section is posterior to 

 the head-fold; on this side the secondary fold is 

 seen as a high arch of ectoderm, with a thick mass 

 of entoderm beneath it. 



Figure 6c represents a section which passes 

 back of the head-fold on both sides. The second- 

 ary folds (sf) are seen as a pair of ectodermal arches 

 continuous with each other in the middle line of the 

 embryo. The ectoderm of the folds is much 

 thickened and gradually becomes thinner distally. 

 On the right the entoderm shows the same thicken- 

 ing {en) that was shown on the left side of the 



