THE PROCESS OF FERTILIZATION 287 



the nuclear membrane and a fluid content in which one or more 

 nuclear bodies or nucleoli float. But this does not by any means 

 exhaust what can now be recognized in the structure of the nucleus, 

 and the most important constituents are not even among these, for 

 recent researches, especially those of Hacker, have shown that the 

 nucleolus or the nucleoli, to which there was formerly an inclination 

 to attach a very high importance, must be regarded as only transient 

 formations and not living elements — in fact, as mere collections of 

 organic substance — ' bye-products of the metabolism,' which at a 

 definite time, that is just before the division of the nucleus, disappear 

 from the nuclear space and are used up. We now know that in the 

 resting cell, that is, in the cell which is not in the act of dividing 

 (Fig. 74, A), a very fine network of pale threads, often very difficult 

 to make visible, fills the whole nuclear cavity, like a spider's web or 

 the finest soap bubbles, and that in this so-called nuclear framework 

 there are embedded granules of rounded or angular form (A,chr) 

 which consist of a substance which stains deeply with such pigments 

 as carmine, hsematoxylin, all aniline dyes, &c., and which has there- 

 fore received the name of chromatin. Often, indeed generally, 

 these granules are exceedingly small, but sometimes they are bigger, 

 and in that case they are less numerous and more easily made visible ; 

 in all cases, however, they are in a certain sense the most important 

 part of the nucleus, for we must assume that it is their influence 

 which determines the nature of the cell, which, so to speak, impresses 

 it with the specific stamp, and makes the young cell a muscle-cell or a 

 nerve-cell, which even gives the germ-cell the power of producing, 

 by continued multiplication through division, a whole multicellular 

 organism of a particular structure and definite differentiation, in 

 short, a new individual of the particular species to which the parents 

 belong. We call the substance of which these chromatin granules 

 consist by the name first introduced into science by Nageli, though 

 only to designate a postulated substance which had not at that time 

 been observed, but which he imagined to be contained within the 

 cell-body— by the name Idwplasm, that is to say, a living substance 

 determining the individual nature (elSo? = form). I am anticipating 

 here, and I reserve a more detailed explanation until I can gradually 

 bring together all the facts which justify the conception I have just 

 indicated of the ' chromatin grains ' as an ' idioplasm,' or, as we may 

 also call it, a ' hereditary substance.' 



That this chromatin must be something quite special we see from 

 the processes of cell and nuclear division, which I shall now briefly 

 describe. 



