134 MCMURRICH. [Vol. XI. 



far that they act independently of them. In fact we see this 

 same phenomenon in various degrees of perfection in the differ- 

 entiation of the mesoderm from the endoderm, for phylo- 

 genetically the mesoderm is derived from the endoderm, as 

 may be seen for instance in the group of the Turbellaria and 

 as is indicated in the ontogeny of many forms. In some cases, 

 e.g., the Echinoderms, the two layers separate perfectly only 

 after they have passed into the blastocoel ; in other cases they 

 are differentiated from one another in the blastula and pass 

 inward together, while in other cases again the migration or 

 invagination of the endoderm may precede that of the meso- 

 derm, or the mesoderm, either in whole or in part, may pass 

 inward before the endoderm. To speak of the origin of the 

 mesoderm in one case from the ectoderm and in another from 

 the endoderm shows a want of appreciation of the phylogenetic 

 significance of the mesoderm and of the possibilities which 

 result from differentiation. 



Primarily, then, in the Crustacea there was the formation of 

 a blastula whose cavity was more or less completely filled with 

 food yolk, and there was a differentiation of mesendodermal 

 cells by immigration, the distinction of mesoderm from en- 

 doderm only supervening later. I do not mean to assert that 

 the more remote ancestors of the Crustacea may not have 

 shown an invagination of the epibolic or even of the embolic 

 type ; in reality the distinction between invagination and immi- 

 gration is so slight that one may readily become converted into 

 the other, and the only question of interest in connection with 

 the two phenomena is as to which is the most primitive phylo- 

 genetically. I see no reason to withdraw from the position I 

 have taken on this question in earlier papers, in which I an- 

 nounced myself as a supporter of Metschnikoff, and do not in- 

 tend to discuss the question here. I believe, firmly, however, 

 that the cases of invagination to be found within the Crustacea 

 are secondary. 



If these cases be examined, some interesting relations are to 

 be found, relations which involve a correct appreciation of the 

 significance of the vitellophag cells. I have already stated my 

 opinion that these structures are to be regarded as mesoderm 



