77/ E NUCLEUS 69 



("vesicular nucleus"). Since "the chromatic particles are the only con- 

 stituents of the cell which maintain persistently and uninterruptedly 

 their existence throughout the whole life cycle of living organisms uni- 

 versally," Minchin (1916) believes that the earliest living things, which 

 he calls " Biococci/' were minute particles of a chromat in-like substance. 

 These were the ancestors of the present chromatin grains and find their 

 nearest modern representatives in certain pathogenic Chlamydozoa. Ac- 

 cording to this view the cytoplasm was differentiated later in the evolu- 

 tion of the cell, whereas the more general view probably is that chromatin 

 and cytoplasm were coexistent as two substances in cells from theearliesl 

 known stages (Wilson). 1 



The Function of the Nucleus. — It may be said without reservation 

 that the nucleus dominates the morphological and physiological changes 

 in the cell. Although the type of organization formed by a nucleus in 

 combination with cytoplasm is required for the carrying on of cell activity, 

 it is nevertheless evident from a huge mass of accumulated observations 

 that in the nucleus is to be found the center of control for both the func- 

 tional activities and for cell reproduction (cell-division). Many years 

 ago Claude Bernard (1878) pointed out that while the cytoplasm is the 

 seat of destructive metabolism, the nucleus is the seat of constructive 

 metabolism, this physiological role offering "the key to its significance as 

 the organ of development, regeneration, and inheritance " (Wilson). The 

 inability of a cell deprived of its nucleus to carry on synthetic metabolism 

 in any complete manner has often been noted, though such a cell may not 

 perish for some time. The mammalian erythrocyte, for example, loses 

 its nucleus at an early stage and may continue to exist in the enucleate 

 state for from 15 to 30 days (Hunter, Quincke). Klebs found that 

 enucleate cells of Spirogyra may continue for some time to form starch. 

 But such cells are apparently unable to divide or to increase their bulk- 

 by the elaboration of new cell substance. Many ordinary act Lvities, such 

 as cell wall formation (Townsend 1897; Gerassimow 1899, 1901), fail to 

 occur. From such observations it is concluded that the nucleus is acc- 

 essary for the synthetic processes associated with growth and reproduc- 

 tion. This conclusion is supported by the facts of regeneration. 



The role of the nucleus in regeneration was strikingly shown by the 

 well known experiments of Gruber (1885) and F. H. Li Hie (1896) on SU ><- 

 tor. This unicellular organism, which has a nucleus like a string of beads, 

 may be cut into fragments: any fragment containing a portion of the 

 nucleus has the power of regenerating a complete new animal, whereas 

 enucleate fragments, although they may live for a little time, undergo do 

 regeneration and eventually perish. 



'For more complete descriptions of the nuclei of Protista the works of Wilson 

 (1900) and Minchin (1912) should be consulted. The behavior <>f such nuclei at the 

 time of coil-division is briefly described in Chapter X of tliis book. 



