REPRODUCTION 209 



phological evidence that all granules are not thus equally distributed 

 is furnished by all budding and spore-forming types, and by forms 

 like Dileptus gigas or Holosticha multinucleata, where the large 

 chromatin granules, while still in the process of division, are carried 

 bodily to one or the other daughter cell (Fig. 46, p. 92). 



Reproduction whereby a type of organism is perpetuated and 

 distributed, is thus preeminently a process of division. In the last 

 analysis cell division is the only kind of reproduction known. 

 Potential individuals are contained in every germ cell, but germ 

 cells, like other cells, are formed by division and it follows that every 

 female reproduces as many potential offspring as eggs. Develop- 

 ment of such eggs, however, is usually dependent upon fertilization, 

 which is quite a distinct phenomenon, accessory to reproduction 

 and necessary in most animals, but not itself reproduction. In the 

 present chapter only a summary of the more obvious phenomena of 

 reproduction will be described, leaving the problems associated 

 with fertilization for treatment in a later section (see Chapter VIII). 



It is division of the grand aggregate of protoplasmic substances, 

 /. e., division of the cell itself, that is usually described as reproduc- 

 tion of the Protozoa. Such reproductions are usually classified as 

 division, budding or gemmation, and sporulation, the inference 

 being that these are different modes of reproduction. In reality, 

 however, they arc different types of reproduction by division, and 

 such modifications would be expressed better by the terms equal 

 division, unequal division, and multiple division. 



I. EQUAL DIVISION AND EVIDENCE OF REORGANIZATION. 



In the ordinary metabolic processes of an active protozoon there 

 is evidence of a cumulative differentiation which indicates a differ- 

 ence in organization between a young cell immediately after division 

 by which it is formed and the same cell when it is mature and ready 

 itself to divide. Child (1916) mainly from experiments with cells 

 of the Metazoa, came to the conclusion that "senescence con- 

 sists in a decrease in metabolic-rate determined by the change 

 in, and the progressive accumulation of, the relatively stabile 

 components of the protoplasmic substratum during growth, develop- 

 ment and differentiation" (loc.cit. p. 333) . He further suggested that 

 in every cell division in unicellular animals, with the accompany- 

 ing processes of reorganization, there is some degree of rejuven- 

 escence and, if such rejuvenescence balances the cumulative differ- 

 entiation, continued life of the organisms by division alone may go 

 on indefinitely. By proper conditions of the environment it is ■ 

 conceivable that such a balance may be established. On such an 

 hypothesis it is possible to account for the continued vitality of 

 animal flagellates in which fertilization processes are unknown, for 

 14 



