THE CELL AND LfS CONSTITUENT STRUCTURES. 143 



broader outlines. Nevertheless, we know, as a matter ot 

 fact, that Ascaris will 7w/ serve as a general type ; we know 

 that differences exist between it and many other organisms in 

 almost every detail, and that only the final result is the same. 

 It is obvious, then, that clearness of differentiation alone will 

 not serve as a criterion of fundamental importance in the 

 case of any given structure, and, perhaps, put in this bald 

 way nobody would expect it would. I have mentioned 

 Ascaris, because it has come to be a familiar object to many 

 people — all the principal phenomena of its nuclear division 

 and of fertilisation can readily be seen without employing 

 any of the more refined methods of technique. But I might 

 equally well have taken the Salamander, which has served 

 as the chief object of Flemming's epoch-making researches, 

 and I may the more specially mention the Salamander in 

 this place, since it differs in so very many details from 

 Ascaris. 



Having cleared the way thus far, I now propose to con- 

 sider some of what we may perhaps regard as the chief 

 groups of phenomena associated with nuclear division which 

 are known at the present time. In this way I believe we 

 shall be best able not only to estimate the relative import- 

 ance of the several structures involved, and to understand 

 their mutual interaction during the process of karyokinesis, 

 but we shall further have acquired a material basis from 

 which to criticise the views now current as to the way in 

 which the fact of heredity is to be harmonised with the 

 ■complex processes proceeding within the cells themselves. 



For it is to the nucleus and the protoplasm that we must 

 turn for the essential factors which determine the structure 

 and govern the organisation of living beings. But we meet 

 with differences of opinion at the very threshold of our 

 inquiry, as to the share allotted to the nucleus and to the 

 cytoplasm respectively in initiating the process. Some say 

 the stimulus comes from the protoplasm, some say it comes 

 from the nucleus, a third party ascribes it to that veritable 

 Deux €x machina the centrosome ; and Boveri, if I under- 

 stand him rightly, postulates yet another organ, a specialised 

 mass of protoplasm in the vicinity of the centrosome, which 



