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THE AMERICAN NATURALIST 



[Vol. L 



stance or as discrete grains, and the whole marked off 

 sharply from the surrounding cytoplasm, with or without 

 a definite limiting membrane. There is, however, one 

 point in which the nucleus exhibits a progressive evolu- 

 tion of the most important kind. I refer to the gradual 

 elaboration and perfection of the reproductive mechan- 

 ism, the process whereby, when the cell reproduces itself 

 by fission, the chromatin-elements are distributed between 

 the two daughter-cells. 



The chromatin-constituents of the cell are regarded, on 

 the view maintained here, as a number of minute gran- 

 ules, each representing a primitive independent living 

 individual or biococcus. To each such granule must be 

 attributed the fundamental properties of living organisms 

 in general ; in the first place metabolism, expressed in con- 

 tinual molecular change, in assimilation and in growth, 

 with consequent reproduction ; in the second place specific 

 individuality. As the result of the first of these proper- 

 ties the chromatin-granules, often perhaps ultra-micro- 

 scopic, may be larger or smaller at different times, and 

 they multiply by dividing each into two daughter-gran- 

 ules. As a result of the second property, chromatin- 

 granules in one and the same cell may exhibit qualitative 

 differences and may diverge widely from one another in 

 their reactions and effects on the vital activities of the 

 cell. The chromatin-granules may be either in the form 

 of scattered chromidia or lodged in a definite nucleus. 

 When in the former condition, I have proposed the term 

 chromidiosome 25 for the ultimate chromatinic individual 

 unit ; on the other hand, the term chromiole is commonly 

 in use for the minute chromatin-grains of the nucleus. 

 The terms chromidiosome and chromiole distinguish 

 merely between the situation in the cell, extranuclear or 

 intranuclear, of the individual chromatin-grain or bio- 



25 " Introduction of the Study of the Protozoa," Arnold, 1912, p. 65. 



