134 Darwin, and after Darwin. 



growing consensus of authoritative opinion, that the 

 chromatin fibres are the seats of the material of 

 heredity, or, in other words, that they contain those 

 essential elements of the cell which endow the 

 daughter-cells with their distinctive characters. There- 

 fore, where the parent-cell is an ovum, it follows from 

 this view that all hereditary qualities of the future 

 organism are potentially present in the ultra-micro- 

 scopical structure of the chromatin fibres. 



As I shall have more to say about these processes 

 in the next volume, when we shall see the important 

 part which they bear in Weismann's theory of 

 heredity, it is with a double purpose that I here 

 introduce these yet further illustrations of them upon a 

 somewhat larger scale. The present purpose is merely 

 that of showing, more clearly than hitherto, the great 

 complexity of these processes on the one hand, and, 

 on the other, the general similarity which they display 

 in egg-cells and in tissue-cells. But as in relation to 

 this purpose the illustrations speak for themselves, I 

 may now pass on at once to the history of embryonic 

 development, which follows fertilization of the ovum. 



We have seen that when the new nucleus of the 

 fertilized ovum (which is formed by a coalescence of the 

 male pronucleus with the female) has completed its 

 karyokinetic processes, it is divided into two equal 

 parts ; that these are disposed at opposite poles of the 

 ovum ; and that the whole contents of the ovum are 

 thereupon likewise divided into two equal parts, with 

 the result that there are now two nucleated cells within 

 the spherical wall of the ovum where before there had 

 only been one. Moreover, we have also seen that a 



