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 (ez). 
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 2d 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. 2¢). 
Figure 2e is a section through the region just 
