2']2 The Alligator and Its Allies 



in thickness between the ectoderm and entoderm. 

 It shows laterally a slight separation to form the 

 body cavity. 



Figure ii// is about ten sections posterior to 

 Figure iig, and differs from it chiefly in that the 

 notochord {}it) is continuous with the lower side 

 of the medullary canal (mc), though still distinct 

 from the underlying entoderm {eii). 



Figure 1 1 /, four sections farther from the head, 

 shows the same greatly thickened ectoderm (fc) 

 with the same break {ec') in the middle line. The 

 section is posterior to the notochord and passes 

 through the anterior edge of the blastopore or, as 

 it may now perhaps better be called, the neuren- 

 teric canal. The cells of the medullary wall are 

 continuous with those of the entoderm. The 

 mesoderm {mes) is still distinct from the other 

 germ layers. 



Figure i \j is the next section posterior to the one 

 just described and differs from it only in showing 

 the actual opening of the neurenteric canal {nc) 

 into the medullary canal {mc). The medullary 

 canal extends, with gradually diminishing caliber, 

 for about fifteen sections posterior to the point 

 at which the neurenteric canal empties into it. 

 The mesoblast {mes) is so closely attached to the 

 lower wall of the neurenteric canal that it seems 

 to be actually continuous with it. 



For a considerable distance posterior to the 

 end of the medullary canal we find the structure 



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