SYNGAMY AND SEX IN THE PROTOZOA 131 



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 r I 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 cf 

 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 



