234 The Alligator and Its Allies 



in the median region, while it gradually thins out 

 laterally. Closely underlying this ectoderm is a 

 thin sheet of irregular cells, the entoderm (en). 



Figure 2c is about one fifth of the length of the 

 blastoderm posterior to the preceding and rep- 

 resents approximately the same conditions, except 

 that there is an irregular thickening of the entoderm 

 in the median region (en) . This thickening appar- 

 ently marks the anterior limit of the mesoderm, to 

 be discussed shortly. 



Figure 2 d represents the condition of the blasto- 

 derm throughout about one third of its length, 

 posterior to the preceding section. The somewhat 

 regular folds in the ectoderm (ec) are probably not 

 medullary folds, but are such artificial folds 

 as might easily be produced in handling the delicate 

 blastoderm. The thickening of the entoderm, 

 noticed in the preceding figure, is here more 

 sharply defined, and as we pass toward the blasto- 

 pore becomes separated somewhat from the ento- 

 derm proper as a middle layer or mesoderm (Fig. 

 2e, mes). It would thus seem, from a study of 

 these sections, that most of the mesoderm is 

 derived from the entoderm. In fact, all of the 

 mesoderm in front of the blastopore seems to 

 have this origin, for it is not until the anterior 

 edge of the blastopore is reached that there is any 

 connection between the ectoderm and entoderm 

 (Fig. 2e). 



Figure 2c is a section through the region just 



