THE PROTOZOAN LIFE CYCLE. 235 



In Tetraniitus , Cerconionas, or other similar flagellates with a 

 firm contour and a definite shape, the organisms in the period 

 of adolescence become plastic or even amoeboid, and like Parauic- 

 ciiun they conjugate while in this condition. 



Among the morphological changes that occur during this 

 period of adolescence none are more subtle or more difficult to 

 interpret than those of the nucleus ; indeed, we are here brought 

 face to face with one of the fundamental problems of modern 

 biology - - the maturation phenomena. While these are undoubt- 

 edly general biology or cell problems, there is another phenom- 

 enon connected with the nucleus of protozoa at this period which, 

 despite the ingenious analogies of Goldschmidt, has no satisfac- 

 tory simulacrum in metazoa, --the formation of chroinidia. 

 Using the term idiochromidium proposed by Mesnil to designate 

 the distributed chromatin prior to gamete formation, we see in 

 this material a practically characteristic sexual substance which, 

 appearing prior to conjugation, belongs primarily to this period 

 of adolescence. In all those forms in which multiple fragmenta- 

 tion has been described, the fragments become the nuclei of con- 

 jugating gametes (examples in Coccidiitin schiibergi and Eutainccba 

 according to Schaudinn). In rhizopods, however, especially in 

 the testacea, this nuclear distribution begins at an earlier period 

 in the life history, and the idiochromidium is characteristic of the 

 ordinary vegetative forms (examples in Difflugia [Zueltzer], 

 Centropyxis [Schaudinn], etc.). This chromatin differentiation 

 and distribution indicates a curious change in the chemical 

 balance of nucleus and cytoplasm during the period of adoles- 

 cence ; a change which may appear earlier or later in different 

 types, the earliest appearance being seen in the case of infusoria 

 where the differentiation into vegetative and germinal nuclei 

 occurs immediately after conjugation. In the majority of cases, 

 however, the appearance of the idiochromidium and the various 

 stages leading up to its formation may be taken as evidence of 

 advancing age of the series of individuals and as a token of the 

 near completion of the cycle. 



So widespread is this nuclear phenomenon and so well estab- 

 lished in protozoa studies that we are justified in some cases in 

 arguing from this phenomenon alone that adolescence and the 



