THE CELL AND CELL DIVISION 



59 



throughout the cell (Fig. 26, A). Such a condition indicates strongly 

 that the nuclear and cytoplasmic parts of the cell have arisen through 

 the gradual differentiation of a common protoplasmic basis. In cell 

 division each of these chromatic bodies may first divide into two 

 (Dileptus), though the members of the resulting pair are not distributed 

 to different daughter cells, for the accompanying division of the cell 

 is completed without any rearrangement of the chromatin granules 

 (Fig. 26, B). In other forms with distributed nuclei (Tetramitus), the 

 scattered granules collect about an active kinoplasmic organ termed 

 the division center (Fig. 27) ; this divides, and the two products separate, 



FIG. 27. Cell division in the Flagellate, Tetramitus. After Calkins. A 

 Vegetative condition showing scattered chromatin granules (distributed nucleus) 

 and division center. B. Collection of chromatin granules preparatory to divi- 

 sion. C. Fission of the division center. D. Separation of the division centers 

 accompanied by the daughter groups of granules (nuclei). 



each accompanied by a group of chromatic granules which are then 

 redistributed equally to the daughter cells, although no definite mitotic 

 figure is formed. Even when a definite nucleus does come to be estab- 

 lished, much of the chromatin of the cell may not be contained within 

 it but may remain distributed (Heliozoa, Radiolaria) . Finally, of course, 

 all of the chromatin becomes contained within one or more definite 

 nuclear structures which may be simple spherical bodies, or they may 

 show considerable complication and variation in form. A definite 

 nuclear membrane may be absent at first, as in Chilomonas and 

 Trachelomonas, though it is formed in practically all cases where the 

 chromatin granules form definite nuclear groups. Within the nucleus 

 the chromatic substance may not be definitely organized into chromo- 

 somes, or these bodies may appear only in certain divisions associated 

 with gametic reproduction - (Paramcecium) ; in some Protozoa, however, 

 definite chromosomes are typically established and become clearly 

 marked during each mitosis (Fig. 28). 



