TliANSACTIONS OF SECTION D. 597 



is indicated by nuclear processes, such as the formation of cbromidia or of dis- 

 tributed chromatin which, as in rhizopods, may persist through many generations 

 of vegetative individuals, or, as in sporozoa, it may be formed only at the end of 

 of vegetative life. In both cases the outcome is the same, and the distributed 

 granules of chromatin form the nuclei of the conjugating gametes. These cell 

 changes are particularly important in a life-cycle, indicating, as they do, the 

 approaching critical period in the history of the race. 



We are fully justified, it seems to me, at least for taxonomic purposes, in 

 regarding the ' individual ' in protozoa, not as a single cell, but as the whole 

 congeries of cells which are formed from the time of the fertilised gamete to 

 natural death from old age. This sequence of stages constitutes the life-cycle, and 

 the morphological characteristics of each phase must be taken into account to get 

 the only adequate basis for protozoan taxonomy. 



As in metazoa, so in protozoa, we can make a clear distinction between the 

 history of the individual in this taxonomic sense and the history of the race, and 

 we can distinguish vital processes that are characteristic of each. The ordinary 

 phenomena of vegetative life of the cell — metabolism in all its phases — have to do 

 with growth and multiplication, and are functions belonging to the life-cycle in 

 its limited sense. The race, which should be considered as a succession of life- 

 cycles, is dependent upon quite a different set of vital activities which are bound 

 up with the maturation and fertilisation of the germ cells. The problems involved 

 here have an interest quite apart from those of the life-cycle, and are no more 

 intimatelyconnected with the phenomena of vegetative life than are they connected 

 in metazoa with growth and differentiation. In any adequate account, however, 

 of a protozoon, these phenomena of the species must be considered, and the biolo- 

 gical principles underlying them should be studied even more exhaustively than 

 any of the vegetative processes, for upon them depends the continuity of the species. 

 Two or three points relating to these more deeply-lying racial problems have 

 recently developed from my culture-experiments, and relate for the most part 

 to Paramecium, 



Although maturation processes have been generally recognised in protozoa, 

 there has been but little work done that can compare even remotely with the 

 elaborate researches on the origin of the maturation chromosomes in the metazoa, 

 Schaudinn, indeed, has described the reduction in the number of chromosomes in 

 Trypanosoma noctuce i'rom eight to four, and speaks of four ' tetrads ' in the 

 nucleus of the female cell. The origin of the tetrads, however, is not made out. 

 The origin of the maturation chromosomes in raramecium caudatum has been 

 worked out this last year by one of my students. Miss S, W. Cull and myself. 

 The curious and enigmatical ' crescent form ' assumed by the micronucleus is 

 proved to be the stage of synapsis, the chromosomes becoming double at this time 

 by union side by side in typical parasynapsis. The two maturation divisions 

 which follow this preparation are difficult to follow, and the results of our study 

 on this point are not quite ready for publication, 



A second point that we have made out relates to an equally important 

 biological principle, viz., the significance of fertilisation. We found that wliile 

 renewal of vitality Avas effected in 70 per cent, of all cases of 'wild' conjugating 

 paramecia, only 12 per cent, were successful in conjugating individuals that had 

 been under cultivation for some time. It was proved that diverse ancestry is not 

 a necessary condition, one endogamous ex-conjugant being carried through 370 

 generations. We also made out that in this supposedly isogamous conjugation 

 there is a definite indication of incipient fertilisation, as in the case of sexually 

 differentiated protozoan or metazoan germ cells, one of the ex-conjugants being 

 much more vigorous than the other. Thus in 41 per cent, (of ninety-three pairs) 

 of the ex-conjugants, only one of the originallconjugating pair was represented by 

 pi-ogeny at the end of a month ; while in 69 per cent, the progeny of one were 

 twice as numerous as those of the other, 



A third point that comes from the study of these Paramecium cultures relates 

 to the renewal of the cycle or reorganisation without conjugation. Partheno- 

 genesis is known in Trypanosoma noctum and in Plasmodium vivax through the 



