26 THE GERM-PLASM 



become active again at the next cell-division. Each separated 

 half of the nucleus now constitutes a daughter-nucleus, in which 

 it immediately breaks up, and becomes scattered in the form of 

 minute granules in the delicate nuclear network, so that finally 

 a nucleus is formed of exactly the same structure as that with 

 which we started. Similar stages to those which occur in the 

 aggregation of the chromatin substance in the mother-nucleus 

 preparatory to division are passed through during the separation 

 of the daughter-nuclei, but in the reverse order. 



It is evident, as Wilhelm Roux was the first to point out, 

 that the whole complex but wonderfully exact apparatus for the 

 division of the nucleus exists for the purpose of dividing the 

 chromatin substance in a fixed and regular manner, not merely 

 quantitatively, but also in respect of the diffej-ent qualities which 

 must be contained in it. So complicated an apparatus would 

 have been unnecessary for the quantitative division only : if, 

 however, the chromatin substance is not uniform, but is made up 

 of several or many different qualities, each of which has to be 

 divided as nearly as possible into halves, or according to some 

 definite rule, a better apparatus could not be devised for the 

 purpose. On the strength of this argument, we may therefore 

 represent the hereditary substance as consisting of different ' qual- 

 ities. "^ The same conclusion is arrived at on purely theoretical 

 grounds, as will be shown later on when we follow out the con- 

 sequences of the process of amphimixis. 



For the present it is sufficient to show that the complex 

 mechanism for cell-division exists practically for the sole pur- 

 pose of dividing the chromatin, and that thus the latter is 

 without doubt the most important portion of the nucleus. Since, 

 therefore, the hereditary substance is contained within the 

 nucleus, tJie chromatin must be the hereditary substance. 



De Vries^s objection to this view is, in my opinion, only an 

 apparent one ; for it has not been asserted that ' the nucleus 

 alone is the bearer of the hereditary characters,' as de Vries 

 thinks, but that the nucleus alone contains the hereditary sub- 

 stance, or that substance which is capable of determining not 

 only the character of a particular cell, but also that of its descend- 

 ants. This is never contained in the cell-body, but always in 

 the nucleus in multicellular organisms, and doubtless the same 

 holds good for unicellular beings. It is quite possible that in 

 certain lower Algae a few of the structures in the cell — such 



