272 The Alligator and Its Allies 



in thickness between the ectoderm and entoderm. 

 It shows laterally a slight separation to form the 

 body cavity. 



Figure nh is about ten sections posterior to 

 Figure ng, and differs from it chiefly in that the 

 notochord (nt) is continuous with the lower side 

 of the medullary canal (me), though still distinct 

 from the underlying entoderm (en). 



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

 shows the same greatly thickened ectoderm (ec) 

 with the same break (ec f ) 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 ij 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 (me). 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 



