210 THE CONTINUITY OF THE GERM-PLASM AS THE 



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 egg of Diptera, 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 undoubtedly 

 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 embryonic cells are equal in reality as well as 

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

 mines the character of the cell-body, and although every differ- 

 entiation of the latter depends upon a certain structure of its 

 nucleoplasm, it does not necessarily follow that the converse pro- 

 position 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 impossible 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 mole- 

 cular structure of the nucleoplasm need not exercise a transforming 

 influence on the cytoplasm, and we can easily imagine that a long 

 series of changes in the nucleoplasm may 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 che- 

 mical structure is concerned. This suggestion is in accordance 

 with the fact that during the first period of embryonic develop- 

 ment in animals, the cell-bodies do not exhibit any visible differ- 

 ences, or only such as are very slight ; although exceptional in- 

 stances occur, especially among the lower animals. But even 

 these latter (e. g. the difference in appearance of the cells of the 

 ectoderm and endoderm in sponges and Coelenterata) perhaps 

 depend more largely upon a different admixture of nutritive sub- 



