270 McMURRICH. [Vol. II. 



as one of the foundation stones of the germ-layer theory. But, 

 after all, can we directly homologize the embryological and coe- 

 lenterate layers } Are the coelenterate layers morphologically 

 differentiated .? It seems to me that they are not ; every kind 

 of cell, glandular, muscular, sensory, ganglionic, and even 

 nematoblastic, which we find in the ectoderm, occurs also in 

 the endoderm. The Coelentera represent a stage in the evolu- 

 tion of the diploblastic condition, rather than the completion 

 of that condition, and we are assuming too much when we 

 make a direct homology of their ectoderm and endoderm with 

 the epiblast and hypoblast of, let us say, a vertebrate embryo. 

 I have spoken of only two layers in the Coelentera, omitting 

 the mesogloea. This term, now generally accepted for the 

 intermediate layer of the Coelentera, is sufficient reason for so 

 doing, since it implies a lack of homology of the intermediate 

 layer with the mesoderm of higher types. It seems to me, 

 and I have so expressed myself elsewhere, that, if we are to 

 seek for a homologue of the coelenterate mesogloea in higher 

 forms, we must look for it in the limiting membrane which 

 occurs just below the ectoderm. Indeed, a comparison of the 

 mesogloea with the limiting membrane of certain polyclades is 

 exceedingly instructive. 



If, then, we regard the Coelentera as presenting merely 

 an approximation to a diploblastic condition, the distinction 

 between an ectodermal and an endodermal origin of any of 

 their parts becomes relatively of little moment. And, further- 

 more, we need not be surprised to find that structures which 

 in certain antimeres develop from one so-called germ layer, 

 may arise from the other in other antimeres. This may be the 

 case with the glandular streaks, and both those who have 

 spoken in favor of their ectodermal origin and those who have 

 maintained that they were endodermal may have right on their 

 side. The observations of Goette ('93) and Miss Hyde ('94) 

 have given reasons for believing that, in the Scyphomedusae, 

 while the first pair of radial chambers is endodermal, the second 

 pair is ectodermal in origin. If such variation occurs in this 

 group in connection with such fundamental structures, surely 

 we may meet with variations in the origin of the glandular 



