No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 133 



These results, however, since Samassa's observations, stand in 

 need of confirmation before they can be accepted. In Ccto- 

 cJiilus Grobben ('8l) describes an invagination of the endoderm 

 and a distinct differentiation of the mesoderm from this, but in 

 description after description of Crustacean development one 

 reads of a solid endodermal plug, a condition which seems to 

 be of the most frequent occurrence. 



However, not unfrequently it is possible to distinguish a 

 differentiation of the blastoporic cells, if the term blastopore 

 can be applied here, and in this respect the phenomena de- 

 scribed in preceding pages are exceedingly interesting. In 

 CyniotJioa, Ligia, and the other Oniscidas, and practically in 

 Asellus, no differentiation can be made out, but mjaera a well- 

 marked differentiation of a portion of the mesoderm, namely of 

 the vitellophags, occurs. The differentiation of the endoderm 

 can be disregarded, since it is one of position only and does not 

 persist, so that we have in that form an anterior mesendo- 

 dermal mass and a posterior mesodermal group of vitellophags. 

 Such a differentiation is peculiar ; it is not a differentiation of 

 endoderm and mesoderm, but a specialization of a certain part 

 of the mesoderm from the remainder, and it is interesting as 

 indicating a process of precocious segregation which may be 

 carried to extreme lengths, and to which the mesoderm, on 

 account of the multiplicity of organs which arise from it, is 

 especially susceptible. Thus the remarkable differentiations of 

 teloblast cells which are found in such cases as Ltunbricns 

 (Wilson, '89) and Clepsine (Whitman, '78) are to be regarded as 

 cases of this kind. From the general ectoderm are differ- 

 entiated two teloblasts which give rise to the ventral nerve 

 cord ; and from the general mesoderm are differentiated the 

 nephroblasts. We find, too, the interesting peculiarity that 

 the nephroblasts pass into the blastocoel at a later period than 

 do the mesoblasts, and the conclusion has been drawn that the 

 nephridia therefore are of ectodermal origin in these cases. Is 

 there any necessity for such a conclusion } It seems to me 

 that the phenomenon is simply the culmination of the process 

 of differentiation of portions of the germ-layer, the segregation 

 of the nephroblasts from the mesoblasts having proceeded so 



