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



same species. To what extent this apparent discontinuity of 

 the protoplasm of the various cells is a reality, and how far it 

 is of importance for the complete histological differentiation of 

 the cells I do not propose to discuss, but wish to point out that 

 the existence of a syncytiiun is no bar' to a certain amount of dif- 

 ferentiation. Thus to confine our attention to Jaera, in which 

 early differentiation is most marked, in the eight-celled stage 

 the cell D has been separated off and, as has been seen, gives 

 rise solely to vitellophags, and at the sixteen-celled stage the 

 ectoderm is thoroughly differentiated from the mes-endoderm. 

 In this stage, however, there is no visible difference in the 

 cells, and that there is a differentiation can only be determined 

 by tracing their future history. In the next stage, however, 

 which, as has been shown, is still a syncytium, histological dif- 

 ferentiation is distinct, the vitellophags presenting a very dif- 

 ferent appearance from the mes-endoderm cells, and these from 

 the ectoderm. 



This syncytial differentiation reminds one very forcibly of 

 the differentiation found in the ciliate Infusoria ; it is a differ- 

 entiation which proceeds independently of the existence of cell 

 boundaries, the force which compels it being at present beyond 

 our ken, and not to be regarded, it seems to me, as resident in 

 the nucleus. The fact of the occurrence of cytoplasmic differ- 

 entiation in uninuclear Infusoria stands in opposition to any such 

 view, and a phenomenon which has been described on a preceding 

 page as occurring in the early stages of development of Porcellio 

 and Armadillidiitm seems to demonstrate that cytoplasmic differ- 

 entiation may occur independently of definite nuclear influence. 

 I refer to the remarkable concentration of the peripheral proto- 

 plasm which occurs in ova of these forms at the four-celled 

 stage. (See Figs. 41-44.) The region towards which the 

 concentration takes place is that in which later the blastoderm 

 will be developed and at the time of its appearance the nuclei 

 present in the ovum are still some distance from the surface, 

 embedded in the yolk, and only in the 32-celled stage 

 do they pass into the peripheral protoplasm, the cytoplasm 

 which surrounds them fusing with it. We have to do here 

 with a precocious segregation of a portion of the cytoplasm which 



