SYNGAMY AND SEX IN THE PROTOZOA 151 



stituting the complex body of a Metazoan individual ; the other 

 represented by the single cells of which both soma and germen 

 alike are built up. The phrase " reproduction," whether sexual 

 or non-sexual, as applied to the Metazoa, refers only to the complex 

 multicellular body as a whole, and not to its constituent cells, 

 which reproduce themselves uninterruptedly by fission during the 

 whole life-cycle. 



In the comparison of a typical Protozoan life-cycle with that of 

 the Metazoa, we may start in both cases alike from a single cell- 

 individual which is the result of an act of syngamy. In Protozoa, 

 also, the zygote multiplies, sooner or later, to produce numerous 

 cell-individuals ; but in this case the cells remain separate from 

 one another and independent, so that no multicellular body is 

 produced, except in the colony-building species, nor is there any 

 distinction of somatic and germinal cells, save in rare cases, such 

 as Volvox- (p. 267). In Protozoa the phenomena of vital exhaustion, 

 so-called " senility " (Maupas) or " depression " (Calkins, Hertwig), 

 appear to be as inevitable as in the cells of the Metazoan body 

 (see pp. 135 and 208, infra) ; but if the derangement of the bodily 

 functions and the vital mechanism has not gone too far, the organism 

 is able to recuperate itself by self-regulative processes, of which 

 the most important and most natural are those involved in the 

 normal process of syngamy. Consequently no cell - individuals 

 among Protozoa are doomed necessarily and inevitably to decadence 

 and death, but all possess equally potential immortality that is 

 to say, the capacity for infinite reproduction by fission under favour- 

 able conditions. The Metazoan individual is represented in the 

 Protozoa only by the entire life-cycle, from one act of syngamy to 

 the next, and not by any living organic individual. 



In the life-cycle of a Protozoon, as there is only one individuality, 

 so there is only one method of reproduction that, namely, of the 

 cell, by fission ; and it must be made clear that the reproduction 

 of the cell-individual is not in any special relation to syngamy 

 in Protozoa, any more than in Metazoa. 



It has been pointed out above that the life-history of a Protist 

 organism consists of alternate periods of growth and reproduction. 

 In those Protozoa in which syngamy has been observed, it is found 

 to take place sometimes at the end of a period of growth and before 

 a period of reproduction, sometimes at the end of a period of 

 reproduction and before a period of growth, and sometimes there 

 may be a difference between the two sexes of the same species 

 in this respect. In the first case, syngamy takes place between 

 full-grown individuals of the species, as in Actinophrys (Fig. 71) 

 so-called macrogamy, which is almost always isogamous. In the 

 second case, syngamy is between the smallest individuals produced 



