THE PROTOZOAN LIFE CYCLE. 239 



species or race. Among the latter are the many processes 

 accompanying fertilization and rejuvenescence, and the phenom- 

 ena here, although not a part of the life cycle any more than 

 sexual processes are a part of the life history of an individual 

 metazoon, are nevertheless dependent on it and can not be omitted 

 in any adequate account of a life cycle, since the succession of 

 cycles, or the race, depends upon them. It is in this field of 

 phenomena that we find the most fascinating aspects of modern 

 protozoa study, for the problems here are general biological 

 problems, and their solution means the illumination of some of 

 the darkest places in biological science. Let me ask your atten- 

 tion for a few minutes, in concluding, to some features of this 

 general subject that have interested me during the past year. 

 They relate to maturation phenomena, to renewal of vitality, after 

 conjugation, and to artificial rejuvenescence in Paramecium, all 

 functions of the species or race rather than of the unit life cycle. 

 One of the deepest problems of general biology, heredity, is 

 bound up with the history of the chromatin in the formation of 

 germ cells, and here in protozoa, as the culminating phenomena 

 of the period of adolescence, we find the same type of matura- 

 tion as in metazoa or the higher plants. At the present time 

 there seems to be no connection whatsoever between the phe- 

 nomenon of chromidium formation and maturation, while the 

 residual masses of chromatin that are left to degenerate and dis- 

 appear in so many protozoa prior to fertilization, are more nearly 

 comparable with the residual chromatin of a germinal vesicle in 

 a metazoon than with maturation of the chromosomes. When 

 the history of the idiochromidium is more perfectly established, 

 we may have further evidence to support the identity of the proc- 

 esses in protozoa and metazoa. The division of the chromidium 

 granules in Amoeba protcus ' which I have elsewhere described in 

 some detail may be the equivalent of the maturation divisions of 

 the chromosomes in germ cells of the metazoa. There is more 

 definite evidence of this similarity in other forms of protozoa. 

 In Trypanosoma noctuce, for example, maturation processes 

 entirely similar to those of the higher animals have been de- 



1 " Evidence of a Sexual Phase in the Life Cycle of A inceba fro/ens," Arch. /. 

 Protist., Bd. V., 1904. 



