208 BIOLOGY OF THE PROTOZOA 



female reproduces as many potential offspring as eggs. Develop- 

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

 which is ciuite a distinct phenomenon, accessory to reproduction in 

 most animals, but not itself reproduction. In the present chapter 

 only a summary- of the more obvious processes of reproduction will 

 be described, leaving the problems associated with fertilization for 

 treatment in a later section (see Chapter XI). 



It is division of the grand aggregate of protoplasmic substances, 

 i. 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 are 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 (see Chapters III and X). Child (1916) mainly 

 from experiments with cells of the Metazoa, came to the conclusion 

 that "senescence consists 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, 

 development and differentiation" (p. 333). He further suggested 

 that in every cell division in unicellular animals, with the accom- 

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

 escence, and if such rejuvenescence balances the ciunulative 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 

 the continued life of many of the higher plants, and for the con- 

 tinued life of the tissue cell cultures in the hands of Carrel and 

 others (see Chapter X). 



In many Protozoa there is unmistakable evidence of such reorgan- 

 ization processes which will be described in the following pages; 

 in many there is no visible evidence, l)ut in such cases and in the 

 absence of other possibilities of reorganization, it is permissible to 

 assume that reorganization processes which escape the most vigilant 

 watchfulness of the observer, do actually occur. For descriptive 

 purposes, and on grounds of expediency, the division phenomena 



