372 PROGRESS OF SCIENCE IN THE CENTURY. 



Oscar Hertwig, and others made it clear that fertil- 

 isation in plants and animals alike is an intimate 

 and orderly union of a spermatozoon and an ovum, 

 — a union in which the two nuclei play a very im- 

 portant part. 



It is generally believed that the paternal and 

 maternal hereditary qualities which are united in 

 fertilisation have their seat in the sperm-nucleus and 

 the ovum-nucleus, especially or exclusively in the 

 readily stainable or chromatin substance of these ; as 

 the ovum is very much larger than the spermatozoon, 

 it evidently supplies most of the initial capital of 

 cell-substance; the spermatozoon, however, contrib- 

 utes, apart from its nucleus, a little body called the 

 centrosome which is now well known in many cases 

 of animal fertilisation, and seems to play an impor- 

 tant part in the process of egg-cleavage; the result 

 of the cleavage is that each daughter-cell gets an 

 equal share of the heritage of chromatin. 



We have alluded to the importance of the idea of 

 genetic continuity — that the germ-cell is a link in a 

 continuous chain of germ-cells; but we must place 

 close beside it the striking fact, which is for some 

 stages visibly demonstrable, that the maternal and 

 paternal chromatin-contributions which come together 

 in fertilisation are distributed equally in the cells of 

 the offspring. 



During the last quarter of the nineteenth century 

 there have been many hundreds of researches on 

 fertilisation, and there is perhaps a larger amount 

 of observational material on this subject than on any 

 other except cell-division, but it must not be sup- 

 posed for a moment that the process is understood. 

 The general tendency, following Hertwig and Stras- 

 burger, is to credit the nuclei with being alone im- 



