24 PROTOZOA 



growth." Other modes of reproduction will be studied later 

 (p. 30), after a more detailed inquiry into the structure of 

 the nucleus and of its behaviour in cell -division. All cell- 

 division is accompanied by increased waste, and is consequently 

 catabolic in character, though the anabolic growth of living 

 protoplasm, at the expense of the internal reserves, may be 

 concurrent therewith. 



Cell-Division 



In ordinary cases of fission of an isolated cell the cell 

 elongates, and as it does so, like other viscid bodies, contracts 

 in the middle, which becomes drawn out into a thread, and 

 finally gives way. In some cases (e.g. that of the Amoeba, Fig. 4) 

 the nucleus previously undergoes a similar division by simple 

 constriction, which is called direct or "amitotic" division. But 

 usually the division of the nucleus prior to cell - division is 

 a more complex process, and involves the co-operation of the 

 cytoplasm ; and we must now study in detail the nucleus and its 

 structure in " rest " and in fission.^ 



We have noted above (p. 6, Fig; 2) the structure of the 

 so-called " resting nucleus," ^ when the cell is discharging the 

 ordinary functions of its own life, with its wall, network of linin, 

 chromatin-granules, and nucleole or nucleoles. The chromatin- 

 granules are most abundant at two periods in the life of the 

 cell, (1) when it is young and fresh from division, and (2) at the 

 term of its life, when it is itself preparing for division. In the 

 interim they are fewer, smaller, and stain less intensely. In 

 many Protista the whole or greater part of the chromatin is 

 densely aggregated into a central " nuclein-mass " or karyosome 



1 The nucleus is regarded by some as equivalent to a central nervous organ for 

 the cell ; by others, such as G. Mann and Verworn, as the chief chemical centre of 

 the cell, and notably the seat of the secretion of the zymases or ferments that play 

 so important a part in its life-work ; for it is found that a Protist deprived of its 

 nucleus can execute its vponted movements, but can neither digest nor grow. This 

 conclusion may appear to be rather sweeping and premature, but we have seen 

 that the changes of surface tension are the direct antecedents of the motions of 

 the cytoplasm, we know that such changes are induced by chemical changes ; and 

 thus the nucleus — if it be the central laboratory to which such changes are 

 ultimately due — would really in a certain sense be a directive centre. 



- The term "resting" is very ill-chosen, for even superficial observation shows 

 that the relative position and characters of the internal structures of such a nucleus 

 are constantly changing with the vital activities and functions of the cell. 



