214 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



causes, viz. that tiiey follow from the constitution of the nucleo- 

 plasm. But that one part may remain unchanged, and that 

 such persistence does, as a matter of fact, occur is shown by 

 the cases above described, in which the germ-cells separate 

 very early from the developing egg-cell. Thus in the ^g^, of 

 Diptcra, the two nuclei which are first separated by division 

 from the segmentation nucleus, form the sexual cells, and this 

 proves that they receive the germ-plasm of the segmentation 

 nucleus unchanged. But during or before the separation of 

 these two nuclei, the remaining part of the segmentation 

 nucleus must have become changed in nature, or else it would 

 continue to form 'pole-cells' at a later period instead of forming 

 somatic cells. Although in many cases the cell-bodies of such 

 early embryonic cells fail to exhibit any visible differences, the 

 idioplasm of their nuclei must undoubtedl}' differ, or else they 

 could not develope in different directions. It seems to me not 

 only possible, but in every way probable, that the bodies of 

 such early embrj'onic cells are equal in reality as well as in 

 appearance ; for, although the idioplasm of the nucleus deter- 

 mines the character of the cell-bod}'', and although every 

 differentiation of the latter depends upon a certain structure 

 of its nucleoplasm, it does not necessarih'' follow that the 

 converse proposition is true, viz. that each change in the 

 structure of the nucleoplasm must effect a change in the cell- 

 body. Just as rain is impossible without clouds, but every 

 cloud does not necessarily produce rain, so growth is im- 

 possible without chemical change, but chemical processes of 

 every kind and degree need not produce growth. In the same 

 manner every kind of change in the molecular structure of the 

 nucleoplasm need not exercise a transforming influence on the 

 cytoplasm, and we can easil}' imagine that a long series of 

 changes in the nucleoplasm ma}^ appear only in the kind and 

 energy of the nuclear divisions which take place, the cell- 

 substance remaining unchanged, as far as its molecular and 

 chemical structure is concerned. This suggestion is in ac- 

 cordance with the fact that during the first period of embryonic 

 development in animals, the cell-bodies do not exhibit any 

 visible differences, or only such as arc very slight; although 

 exceptional instances occur, especially among the lower 

 animals. But even these latter (e.g. the difference in appear- 



