No. 589] THE EVOLUTION OF THE CELL 



13 



sible. I shall deal with some of these types later in my 

 attempts to reconstruct the evolution and phylogeny of 

 the cell. I will draw attention now only to a few salient 

 points. In the Protist cell the chromatin is not neces- 

 sarily confined to the nucleus, but may occur also as e.\t ra- 

 nuclear grains and fragments termed chromidia, scattered 

 through the protoplasmic body; and the chromatin may 

 be found only in the chromidial condition, a definite nu- 

 cleus being temporarily or permanently absent. Fur- 

 ther, when a true nucleus is present in the Protist body, 

 it seldom contains a nucleolus of the same type as that 

 seen in the nuclei of tissue-cells, that is to say, a mass of 

 pure plastin, but in its place is found usually a conspicu- 

 ous body which shows reactions agreeing more or less 

 closely with those of chromatin and which consists of a 

 plastin-basis more or less densely impregnated with 

 chromatin. Such a body is termed a karyosome (or 

 chromatin-nucleolus) to distinguish it from the true nu- 

 cleoli (plastin-nucleoli) characteristic of tissue-cells. Ac- 

 cording as the plastin or the chromatin predominates in 

 the composition of a karyosome, its reactions may re- 

 semble more nearly those of a true nucleolus in the one 

 case, or those of chromatin in the other. The so-called 

 karyosomatic type of nucleus is very common in the Pro- 

 tista, but by no means of invariable occurrence ; in many 

 cases the nucleus consists of a clump of small grains of 

 chromatin, with no distinct karyosome, or with a karyo- 

 some which consists mainly of plastin. Thus two ex- 

 treme types of nuclear structure can be distinguished and 

 may be termed provisionally the karyosomatic type and 

 the granular type, ignoring for the sake of convenience in 

 nomenclature the types of structure transitional between 

 the two; as, for example, types in which a distinct karyo- 

 some is seen together with more or fewer peripherally 

 arranged grains of chromatin. 



In either the karyosomatic or the granular type of 

 Protist nucleus we may find great simplification of the 

 complex type of nuclear structure seen in the tissue-cells 



