THE NUCLEUS 71 



periods of life-history, but is the same, normally, for corresponding 

 phases of the life of the cell ; it can be influenced by external con- 

 ditions, such as food and temperature, and also by internal factors, 

 undergoing changes in a regular manner, in harmony with changing 

 functional conditions of the cell. In cultures of a given species 

 at a lower temperature, multiplication is slower and the organisms 

 grow larger and possess larger nuclei ; with increase of temperature 

 the reverse takes place (compare p. 206, infra). It has also been 

 observed that, in long-continued cultures of Protozoa, periods of 

 active assimilation and multiplication arc followed by periods of 

 depression, during which assimilation and reproduction are at a 

 standstill, even in the midst of abundant nutriment (see especially 

 Calkins, 5). The depression-periods are characterized by an in- 

 crease of the nuclear substance relatively to the cytoplasm, a 

 " hyperchromasy " of the cell, which may lead to the death of the 

 individual unless compensated by the elimination and absorption 

 of part of the nuclear substance (p. 209, infra) ; when the balance 

 has been thus restored, the organism becomes normal and feeds 

 and multiplies again. From this conception of a definite relation 

 between the mass of the nucleus, or rather of the chromatin, and 

 that of the cytoplasm, Hertwig has deduced a number of important 

 consequences to which reference will be made in subsequent chapters. 



The influence exerted by the chromatin upon the life of the 

 organism may be manifested in two ways, which may be termed, 

 for convenience, actual and prospective, respectively. In the first 

 case it regulates the metabolism and functions, both trophic and 

 kinetic, of the cell in which it is contained, and is then commonly 

 termed vegetative chromatin, or trophochromatin. In the second case 

 it may be dormant and inactive in the cell that contains it, remaining 

 latent, as it were, until carried on to future generations in the 

 course of cell-reproduction ; at a later period the whole or a part 

 of this latent chromatin may become active, determining the nature 

 and properties of the offspring, and thus serving as the vehicle for 

 hereditary transmission of the characters of antecedent generations. 

 Such temporarily dormant chromatin is commonly termed genera- 

 tive chromatin, or idiochromatin. It is probable that in all Protozoa 

 the cell-body contains chromatin both in the active and inactive 

 state, the one regulating the vital functions of the living body, 

 the other remaining dormant, in reserve for future generations. 



The validity of this conception, according to which the chromatin 

 present in an organism is regarded as being either vegetative or 

 generative in function, must be tested by its capacity to account 

 for the facts of the development and life-cycle which will be con- 

 sidered more fully in subsequent chapters. There are no means 

 of recognizing and distinguishing vegetative and generative chro- 



