CONCRESCENCE, AND APICAL GROWTH. 



247 



oi apical growth, which form the walls of the infolding, are not formally separated 

 into endoderm, mesoderm, and notochord till a relatively late period. To call 

 this infolding an "archenteron," or "primitive gut," and to then conclude that 

 the notochord was once a part of an alimentary canal, because it is for a brief 

 period united with the definitive endoderm, is no more justifiable than it would be 



Fig. 155. — Photograph""of a half grown Limulus 



to assume that the endoderm is historically derived from an internal skeleton, be- 

 cause for a time it is continuous with the notochord. 



As I pointed out in my first contribution on this subject, in 1889, the only avail- 

 able criterion as to what constitutes endoderm is evidence that it forms the lining 

 of a functional alimentary canal. From this point of view, it is clear that the only 



