THE STRUCTURE OF CELLS 



39 



protoplasm in the periods intervening between two fissions, and are 

 only intimately associated with the central body at the time of cell- 

 division ; in others again, e.g. Chilomonas, they remain constantly 

 grouped in the vicinity of the body to which reference has just 

 been made. No structure has been certainly made out in it, but it 

 has often been compared to, or identified with, the central body 

 present in many of the more highly differentiated Protozoa, such as 

 Euglena, and it has further been likened to the nucleolus and also 

 to the centrosphere of those cells in which these structures have 



FIG. 18. 



Tetramitus. A, resting cell. 

 B, early phase of division, c.b, 

 central body ; eft, chromatin 

 granules. (After Calkins.) 



been found to occur. Indeed, it would seem that there is at least 

 some justification for the latter comparison, inasmuch as it appears 

 at least to discharge functions somewhat similar to those performed 

 by the centrosome though in a very rudimentary degree. 



A distinct advance in differentiation is reached when the 

 chromatic and other constituents of the nuclear apparatus are not 

 only aggregated together, but are also delimited from the rest of 

 the cytoplasm by a wall or membrane. The degree of individuality 

 thus obtained provides a condition favourable to further special- 

 isation, but it seems clear that at any rate the linin framework in 

 which the chromatin is imbedded may be fairly traced back to a 



