244 The Alliijator and Its Allies 



preceding figure. This thickened appearance of 

 the entoderm is due to the fact that the section 

 passes through the anterior limit of a tall fold 

 of that layer, which underlies the similar fold 

 of the ectoderm that has already been described. 

 This secondary fold of the entoderm is seen on 

 the left side of the section. It may be traced 

 through several sections, but soon flattens out 

 posteriorly. 



Figure 6d is a short distance posterior to tne 

 preceding. The secondary folds are here much 

 less pronouncedly arched and the deep groove 

 between them is reduced to a line (/). The ento- 

 derm {eyi) is no longer markedly arched and is 

 closely adherent, along the median plane, to the 

 ectoderm, where there is seen the thickening 

 {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 (ines). 



The section immediately posterior to the one 

 just described is represented in Figure 6e. The 

 line (/) which separated the two secondary 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 (mg), 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 flattening 



