The Cellular Basis 145 



ferent parts of the cytoplasm in definite regions of the cell, and 

 this cytoplasmic localization is sometimes of such a sort that one 

 of the daughter cells may contain one kind of cell substance and 

 the other another kind. 



Cytoplasm Differentiates, Nuclei Do Not. Thus while mitosis 

 brings about a scrupulously equal division of the elements of 

 the nucleus, it may lead to a very unequal and dissimilar division 

 of the cytoplasm. In this is found the significance of mitosis, 

 and it suggests at once that the nucleus contains non-differen- 

 tiating material, viz., the idioplasm or germ-plasm, which is char- 

 acteristic of the race and is carried on from cell to cell and from 

 generation to generation ; whereas the cell body contains the dif- 

 ferentiating substance, the personal plasm or somatoplasm, which 

 gives rise to all the differentiations of cells, tissues and organs 

 in the course of ontogeny. 



Weismann supposed that the mitotic division of the chromo- 

 somes during development was of a differential character, the 

 daughter chromosomes differing from each other at every dif- 

 ferential division in some constant and characteristic way, and that 

 these differentiations of the chromosomes produced the charac- 

 teristic differentiations of the cytoplasm which occur during 

 development. But there is not a particle of evidence that the di- 

 . vision of chromosomes is ever differential ; on the contrary, there 

 is the most complete evidence that their division is always remark- 

 ably equal both quantitatively and qualitatively. If daughter 

 chromosomes and nuclei ever become unlike, as they sometimes do, 

 this unlikeness occurs long after division and is probably the 

 result of the action of different kinds of cytoplasm upon the 

 nuclei, as is true for example, in the differentiation of the chro- 

 mosomes in the somatic cells as contrasted with the germ cells of 

 Ascaris (Fig. 49). In this case Boveri has shown that the nuclei 

 and chromosomes of germ cells and of somatic cells are at first 

 alike whereas the cytoplasm in these cells is unlike; later the 

 nuclei and chromosomes of these two kinds of cells become unlike, 



