442 HEREDITY AND DEVELOPMENT 



tocytes which become spermatozoa. It may be said, we think, 

 in perfect fairness that many of the quite impartial studies 

 on maturation, and the associated reducing-divisions, confirm 

 Weismann's view that there is a segregation of individual chromo- 

 somes (vehicles of several complete sets of hereditary equipments) ; 

 and that there is, therefore, in this process, a modus operandi 

 for new permutations and combinations of ancestral plasms. 

 In other cases, however, this view is not corroborated as yet. 



Fertilisation. — Recent work has forcibly suggested that there 

 are in fertilisation two more or less distinct processes : on the 

 one hand, the process by which the gametes, bearing the 

 hereditary characters, unite to form the beginning of a new in- 

 dividuality ; on the other hand, the process by which the sperma- 

 tozoon supplies some stimulus, prompting the ovum to divide. 

 The first aspect is that of amphimixis, believed by many to be 

 of importance in initiating — and, it may be, also in checking — 

 variations, but in any case effecting the union of hereditary 

 qualities contained in the two gametes. The second aspect 

 is that of mitotic stimulus, believed by some to be afforded by 

 an enzyme — for which the name of " ovulase " has been suggested 

 — and by others to be localised in the sperm-centrosome. It is 

 seen in many cases that equivalent numbers of chromosomes 

 are contributed by the two nuclei ; it is evident that the ovum 

 contributes by far the larger quantity of cytoplasm ; it seems 

 to have been securely demonstrated in some cases that " from 

 the father comes the centrosome to organise the machinery of 

 mitotic division by which the egg splits up into the elements 

 of the tissues, and by which each of these elements receives its 

 quota of the common heritage of chromatin." " Huxley hit 

 the mark two-score years ago when he compared the organism 

 to a web, of which the warp is derived from the female and the 

 woof from the male. What has since been gained is the know- 

 ledge that this web is to be sought in the chromatic substance 

 of the nuclei, and that the centrosome is the weaver at the 



