INTRODUCTION 



Metazoa, nor in plants. The most striking of these is the division 

 of the nucleus in Ciliata and Acinetaria into two unequal and 

 dissimilar portions, the mega-nucleus and the micro-nucleus, which 

 appear to be the portions of the primary nucleus which preside 

 over the somatic (the larger) and reproductive activities (the 

 smaller) respectively. Professor Hickson has made use of this 

 differentiation of the nucleus into two parts in order to establish 

 a higher grade of the Protozoa the Heterokaryota as distinguished 

 from the Homokaryota. 



Amongst those forms, however, which are classed by him as 

 Homokaryota, there are (as he recognises) certain forms amongst 

 the Flagellata which also exhibit a differentiation and segregation 

 of the nucleus, but with functions for the separated elements 

 different from that shown in the Ciliata. This case is that of the 

 formation of a separate nuclear body, the kineto-nucleus, in con- 

 nection with, and apparently controlling the activities of, the large 

 and powerful fiagellum of certain flagellate forms (Trypanosoma, 

 Noctiluca). It seems that the word Heterokaryote would strictly 

 apply to these forms also, although the " heterosis " is not the 

 same as that seen in Ciliata. It would be premature to attempt to 

 introduce a terminology indicating these different specialisations of 

 nuclear structure in the Protozoa until much further study has 

 been given to the subject. It is not at all improbable that researches 

 which are now in progress will in the course of a few years give 

 us, first of all, a better understanding of the chemical nature and 

 activities of the substances which are merely brought into view 

 by colour-staining as form-elements in the nucleus, 1 and secondly, 

 a far more critical knowledge than we at present possess of the 

 rudimentarily aggregated and diffuse stainable matter which is 

 interpreted as " nucleus " in some of the Protozoa, in some of the 

 Cyanophyceae, in Schizomycetes, and in the yeasts and hyphae 

 of lower fungi. 



Whilst therefore recognising the important separation of the 

 Ciliata and Acinetaria effected by having regard to the nuclear 

 structure of those groups and that of the other classes of Protozoa, 

 so far as we at present know them, I am unwilling to emphasise the 

 arrangement of the Protozoa into grades according to their nuclear 

 structure in the present state of knowledge. . I should not wish to 

 go farther at present in grouping the classes of Protozoa than to 

 suggest that they should be considered as diverging lines of descent 

 radiating from a central group which possessed the combination of 

 characters presented at the present day by the simpler Flagellata. 



1 The researches of Professor Macallum of Montreal in this direction will, it 

 may be hoped, be continued and developed. 



