232 THE PROTOZOA 



it, a process regarded by him as a kind of reduction and found in 

 some form or other in other Coccidiida (Adelea, Seidlecki, '99; 

 Coccidium proprium, '98, C. lacasei, Schaudinn, etc.). The macro- 

 gamete is thus essentially a normal individual, in which a reserve 

 store of food is deposited, and in which the quantity of chromatin is 

 reduced. The microgamete, on the other hand, differs widely from 

 the macrogamete, the schizont, and the merozoite. 



The life-history of Coccidium is thus similar to that of Gonium or 

 Pandorina and consists of periods of asexual reproduction, which 

 alternate with periods of so-called sexual reproduction (Fig. 128). 

 This conception of alternation of generations may be extended to all 

 Protozoa, but in only one case ( Volvox) is there a differentiation into 

 germ and somatic cells in the same individual. Volvox affords an 

 interesting intermediate stage between the generalized Protozoa and 

 the specialized forms among the Metazoa and Metaphyta. Here the 

 aggregate is differentiated into somatic and germ cells, although in 

 the early stages there is no difference between them. Some of the 

 latter (parthenogonidia) are sufficiently generalized to form new 

 colonies by asexual reproduction, while others, apparently like the 

 somatic cells, form the sexual reproductive elements ; the remaining 

 peripheral cells are specialized for feeding and motion. 



It has been determined by Kirschner ('79), Carter ('58), and Stein 

 ('78) that some colonies always produce male and others female. In 

 the formation of male elements one of the ordinary cells of Volvox 

 'minor divides into 16 spermatozoids (Kirschner), or from 32 to 128 

 or more (Goroschankin, '75 ; Cohn, '75 ; and Stein, '68). The egg- 

 cell, too, appears at first to differ but slightly from an ordinary cell, 

 but it rapidly grows in size and becomes entirely different. Fertili- 

 zation has been observed in both Volvox minor and V. globator, 

 taking place in the inner spaces of the colonies, where the spermato- 

 zoa work their way to meet the eggs. In no case, however, have the 

 inner processes of fertilization been observed. 



Thus, in all Protozoa, with a single exception, apparently, of Volvox, 

 where the beginnings of specialization are seen, all of the cells of a 

 cycle, or of a colony, are equally capable of conjugation and of 

 rejuvenescence, and the possibility of indefinitely continued life is 

 open to them all. This power, as Weismann first pointed out, dis- 

 tinguishes the Protozoa from all higher animals and plants, where 

 division of labor has resulted in specialization. The majority of the 

 vegetative or somatic cells which form the organs and tissues of 

 higher animals and plants, have, with their specialization, lost the 

 power of rejuvenescence, and when the potential of vitality with 

 which they start is exhausted they become degenerate through old 

 age, and finally die. The germ cells, on the other hand, like the 



