70 THE PROTOZOA 



or grain in the cell cannot be definitely identified as chromatin, in 

 all cases, by any chemical or physical test, but only by its relation 

 to the life and development of the organism as a whole, and more 

 especially to the function of reproduction and the phenomena of 

 sex, as will be shown more fully by means of concrete instances in 

 subsequent chapters. The sum of modern knowledge with regard 

 to the vital activities of living bodies and the life-histories of 

 organisms, whether plants or animals, Protozoa or Metazoa, 

 indicates that the chromatin exercises a regulative and determina- 

 tive influence over the functions and properties of the cell-body. 

 Direct experimental proof of the all -importance of the nucleus for 

 the life of the cell is obtained by cutting Protozoa into pieces, some 

 containing portions of the nucleus, others consisting of cytoplasm 

 alone (p. 210, infra). Those pieces that contain nuclear substance 

 are able to regenerate the lost parts of the body and to perform 

 all the functions of life, and in particular those of assimilation, 

 growth, and reproduction ; those, on the contrary, that contain no 

 portion of the nucleus rapidly lose the power of assimilation, and 

 are unable to regenerate the body, to grow or to reproduce; and 

 though they remain for a time irritable and capable of movement, 

 they soon lose these properties. There are a number of facts which 

 indicate that in the physiological activities of the cell the chief 

 function of the nucleus is the formation of ferments ; it is therefore 

 all-important in regulating the assimilative processes of the living 

 substance (p. 194). 



The conception of chromatin as the directive and regulative centre 

 of the cell-body renders intelligible a number of phenomena con- 

 nected with it, such as the elaborate mechanisms which, as will be 

 described in the next chapter, are gradually evolved and perfected 

 for the exact partition of the chromatin in the reproduction of 

 the cell by division, and the relation of chromatin to the 

 sexual process. Further, the extremely variable nature of the 

 chromatin-substance becomes at once intelligible on this view of 

 its relation to the specific characters and properties of the organism ; 

 for since every species of living being perhaps, even, every in- 

 dividual of the same species differs to a greater or less extent 

 from every other : then, if such differences are determined by the 

 chromatin, it follows that the chromatin must also differ to a 

 corresponding degree in each case, and that consequently uni- 

 formity of character in different samples of chromatin cannot be 

 expected to occur. 



Hertwig (67, 92) considers that a certain quantitative relation of 

 nucleus and cytoplasm is necessary in any cell for the normal 

 continuance of the vital functions. This nucleo-cytoplasmic ratio 

 (" Kernplasma-Relation ") is subject to variations at different 



