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 upona 
somewhat larger scale. The present purpose is merely 
that of showing, more clearly than hitherto, the grcat 
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 betore there had 
only been one. Morcover, we have also seen that a 
