9G THE PROTOZOA 



chromatin-elements ; while the cell-nucleus proper would represent 

 the trophic nucleus deprived of all kinetic elements. 



On the other hand, the condition in amoebae and similar or- 

 ganisms, where the cell appears to contain but a single nucleus 

 which includes the kinetic centres, is explained by supposing that 

 here the kinetic nucleus is eiicapsuled in the trophic nucleus, and 

 is represented by the karyosome with its centriole ; hence the 

 supporters of this theory term the type of nucleus characterized 

 by a large karyosome an " amphinucleus " or " amphikaryon," 

 and, in their descriptions of such nuclei, they speak of the outer 

 nucleus (peripheral zone of chromatin) and the inner nucleus 

 (karyosome). 



The reasons against homologizing the karyosome and the extra- 

 nuclear centrosome have been stated already. Against the theory 

 of binuclearity it may be urged further First, that to regard the 

 protokaryon-type of nucleus seen in the most primitive forms of 

 Sarcodiiia and Flagellata as a secondary condition is a complete 

 inversion of what is, to all appearance, the natural series of evolu- 

 tion of the nuclear apparatus ; secondly, that the binucleate con- 

 dition of trypanosomes and allied forms is clearly, by comparison 

 with other Flagellates, a specialized condition ; the trophonucleus 

 of trypanosomes also contains a karyosome and centriole, and 

 would therefore be an " amphikaryon," on this theory ; thirdly, 

 that the binuclear theory still leaves the centriole as a kinetic 

 centre of achromatinic origin, which is present in both trophonucleus 

 and kinetonucleus of trypanosomes, in both nucleus and central 

 grain of Heliozoa (Wagnerella), etc. All that the binuclear theory 

 is capable of explaining is the secondary elements of the extra- 

 nuclear centrosomic complex. That the centriole is a body of 

 intranuclear origin and formation is shown clearly by its presence 

 in nuclei of the primitive karyosomatic type which arise, not by 

 division of pre-existing nuclei, but by aggregation and organization 

 of clumps of chromidia. It should be added that, in its most recent 

 exposition by Hartmann (61), the theory of binuclearity has 

 undergone considerable modification and restriction. 



Having considered now the structure and composition of the 

 nucleus in its principal types and morphological variations, it 

 remains to attempt to establish a more precise conception as to 

 what exactly is meant by a nucleus. It is evident, in the first 

 place, that the essential component of a nucleus, never absent, is 

 chromatin ; but it is equally clear, in the second place, that a simple 

 mass, or several such masses, of chromatin, do not by themselves 

 constitute a nucleus in the true sense of the word. The word " chro- 

 matin " connotes an essentially physiological and biological con- 

 ception, as it were, of a substance, far from uniform in its chemical 



