14 HEREDITY AND SELECTION IN SOCIOLOGY 



the millions of male cells which crowd round the ovum penetrates 

 the nucleus of the latter. As soon as this successful spermato- 

 zoon has forced its way through the outer covering of the ovum 

 into the nucleus, a thin membrane is formed, which, surrounding 

 the ovum, effectually prevents the entrance of a second sperma- 

 tozoon. The essential point in the process of reproduction is 

 the conjugation or amphimixis of the nuclei of the two cells, of 

 the ovum and spermatozoon. 



The scientific researches of the last forty years of the nineteenth 

 century have gradually disclosed the secrets of that mysterious 

 process of fertilisation by which the continuity of by far the 

 greater part of the animal world is secured. Though we are 

 still far from understanding the ultimate secrets of reproduction, 

 we are better able to appreciate its real nature. The farther 

 research into this mysterious process penetrates, the more com- 

 plicated does it appear, the more delicate and subtle the 

 mechanism, not of fertilisation alone, but of the life of the cell 

 in general. Formerly there was a general impression that the 

 process of cell division, or karyokinesis, was a simple one, merely 

 cutting in half the original cell. But although this " direct " 

 division does indeed appear to occur in some rare cases, the pro- 

 cess of division in the overwhelming majority of cells, and in 

 almost all those which function actively, is exceedingly com- 

 plicated. 



In the first place, the nature of the cell nucleus is extraordinarily 

 complex. We know that the nucleus not only includes a mem- 

 brane and a nuclear fluid, but that in the state of repose a sub- 

 stance known as the " chromatin substance," and readily stain- 

 able with aniline dyes, is to be seen forming a sort of network 

 within it. This chromatin substance is the most important 

 histological element in the nucleus, for it alone is the vehicle of 

 hereditary transmission. The dominant importance of the 

 chromatin as the vehicle of hereditary transmission is proved 

 by the part played by it in the process of cell division. 



