24 THE fJERM-PLASM 



and not the body of the cell, must contain the hereditary sub- 

 stance, the conclusion was drawn that neither the membrane of 

 the nucleus, nor its fluid contents, nor the nucleoli — which 

 latter had been the first to attract attention — could be regarded 

 as such, and that the ' chromatic granules ' alone were important 

 in this respect. As a matter of fact several investigators, — 

 Strasburger. Oscar Hertwig, Kollicker, and myself, — reasoning 

 from the same data, arrived at this conclusion independently, 

 within a short time of one another. 



It will not be considered uninteresting or superfluous to reca- 

 pitulate the weighty reasons which force us to this conclusion, 

 for it is clear that it must be of fundamental importance in a theory 

 of heredity to know for certain what the substance is from which 

 the phenomena which are to be explained proceed. 



The certaintv with which we can claim the ' chromatin gran- 

 ules " of the nucleus as the hereditary substance depends firstly, 

 on the process of amphimixis ; and secondly, on that of nuclear 

 division. We know that the process of fertilisation consists 

 essentially in the association of an equal number of chromatin 

 rods from the paternal and maternal germ-cells, and that these 

 give rise to a new' nucleus from which the formation of the 

 offspring proceeds. We also know that in order to become 

 capable of fertilisation each germ-cell must first get rid of half of 

 its nuclear rods, a process which is accomplished by very peculiar 

 divisions. W^ithout entering into further particulars here, am- 

 phimixis may be described as a process by means of which one- 

 half of the number of nuclear rods is removed from a cell and 

 replaced by an equal number from another germ-cell. 



The manner, however, in which the chromatin substance is 

 divided in nuclear division strengthens the above view of its 

 fundamental nature. This method of division leaves no doubt 

 that it is a substance of the utmost importance. I need only 

 briefly recapitulate the main points of the wonderfully complicated 

 process of the so-called mitotic or karyokinetic cell-division, 

 which follows a definite law even as regards the most minute 

 details. 



When the nucleus is going to divide, the chromatin granules, 

 which till then were scattered, become arranged in a row, and 

 form a long thread, which extends through the nucleus in an 

 irregular spiral, and then divides into portions {chro7)iosomes) of 

 fairly equal length. The chromosomes have at first the form 



