68 INTRODUCTION TO CYTOLOGY 



The apparent discrepancy between this view of bacterial organization 

 and that of Minchin, stated below, will be seen to be largely a matter of 

 terminology. 



It is therefore among the Proitsta that the widest departures from 

 the usual type of nuclear structure are found, certain of them in all prob- 

 ability representing relatively primitive stages in the evolution of the 

 true nucleus. Such an interpretation is evidently to be placed upon the 

 "distributed nuclei" seen in certain bacteria, protozoans, flagellates, 

 and Cyanophyceoe (p. 202), which consist of granules of a material akin 

 to chromatin scattered throughout the cell, sometimes with a limiting 

 membrane of some sort but often with none. It is doubtful if granules 

 scattered with no definite limitations throughout the cell, as in Chcenia 

 teres (Fig. 17, D) or Chroococcus turgidus (Fig. 72, A), should be spoken 

 of collectively as a nucleus. As pointed out at the beginning of this 

 chapter, it seems preferable to certain workers to limit the term to those 

 chromatic aggregations which actually have the characters of a definitely 

 localized organ. In discussing the advisability of so restricting the appli- 

 cation of the term Minchin (1912, Chapter VI) points out that "the word 

 'chromatin' connotes an essentially physiological and biological con- 

 ception . . . of a substance, far from uniform in its chemical nature, 

 which has certain definite relations to the life history and vital activities 

 of the cell. The word 'nucleus,' on the other hand ... is essentially 

 a morphological conception, as of a body, contained in the cell, which 

 exhibits a structure and organization of a certain complexity, and in 

 which the essential constituents, the chromatin particles, are distributed, 

 lodged, and maintained, in the midst of achromatinic elements which 

 exhibit an organized arranegment, variable in different species, but more 

 or less constant in the corresponding phases of the same species." Ac- 

 cording to this interpretation the term "nucleus" would not be applicable 

 to a mass of granules (chromidia) scattered throughout the cell. Minchin 

 states further that " . . .as soon as a mass or a number of particles of 

 chromatin begin to concentrate and separate themselves from the sur- 

 rounding protoplasm, with formation of distinct nuclear sap and ap- 

 pearance of achromatinic supporting elements, we have the beginning 

 at least of that definite organization and structural complexity which is 

 the criterion of a nucleus as distinguished from a chromidial mass." 



Those Protista of the lower (bacterial) grade, in which there are only 

 scattered grains of chromatic material, are looked upon as "non-cellular" 

 in organization by Minchin, who believes that from such a primitive 

 state the "strictly cellular grade of organization has been evolved by 

 concentration of some or all of the chromatin to form a nucleus." In 

 its simplest condition such a nucleus consists of one or more chromatin 

 granules in a sort of vacuole, and is known as a "protokaryon." In other 

 cases the chromatin forms a single large mass at the center of the nucleus 



