20 



THE AMERICAN NATURALIST 



[Vol. L 



least contains, true chroraatin-grains, extranuclear chro- 

 matin for which R. Hertwig's term chromidia is now used 

 universally. It is interesting to note that until the life- 

 history of Arcella was studied in recent times the con- 

 spicuous ring of chromidia was generally overlooked and 

 is not shown in some of the older pictures of the organism. 



If, on the other hand, I make a preparation of some 

 unidentified amoeba occurring casually in pond-water or 

 in an infusion, and find in its cytoplasm certain grains 

 staining in same manner as the chromatin of the nucleus, 

 it is quite impossible, without a knowledge of the life- 

 history of the organism, to assert definitely that the grains 

 in question are or are not true chromidia. They might 

 equally well turn out to be volutin or any other substance 

 that has an affinity for the particular chromatin-stains 

 used in making the preparation. 



The fact that at the present time the only decisive 

 criterion of what is or is not chromatin is supplied only 

 by its behavior in the life-history and its relation to the 

 organism, makes it much easier to identify the chromatin 

 in some cases than in others. In those Protista or cells 

 which contain, during the whole or a part of the life- 

 history, one or more true nuclei, recognizable as such un- 

 mistakably by their structure and their characteristic 

 relations to the reproductive and sexual phenomena of 

 the organism, the chromatin can be identified with cer- 

 tainty. If chromidia occur in the cell-body in addition 

 to true nuclei or even if the nuclei are temporarily absent 

 during certain crises of the life-history and the chromatin 

 occurs then only in the form of chromidia, there is still 

 no difficulty in identifying the scattered chroma tin-grains 

 by the fact that they contribute, soon or later, to the 

 formation of nuclei. 



On the other hand, in the simplest Protist organisms 

 which do not contain definite, compact nuclei recognizable 

 by their structure and behavior, the identification of the 

 chromatin may become correspondingly difficult. In the 

 absence of definite chemical criteria the term chromatin 



