SYNGAMY AND SEX IN THE PROTOZOA 155 



spermatozoon is not indispensable for supplying a developmental 

 stimulus ; an unfertilized ovum can be induced by artificial stimuli 

 of various kinds to start upon a course of development similar to 

 that initiated, under natural circumstances, by fertilization with 

 a spermatozoon. Hence, of the two results produced in the fertiliza- 

 tion of Metazoa, amphimixis alone would appear to be that which is 

 essential and peculiar in the process, and which only fertilization 

 can bring about. 



From the above considerations, amphimixis is regarded by many 

 thinkers as the essence of syngamy, a necessity for the evolution 

 of living beings in that it supplies, by the intermingling of different 

 hereditary tendencies, the conditions required for the production 

 of " innate " variations in organisms in which the germinal substance 

 is shielded from the direct influence of external conditions by its 

 position within a multicellular body. Apart from the question, 

 however, whether any such innate variations exist in the Protozoa, 

 where all cells alike are exposed equally to the direct action of the 

 environment, the criticism has often been made that amphimixis 

 gives only a teleological explanation of the sexual process, and as 

 such cannot be invoked as a causal explanation of its origin. The 

 intermingling of distinct hereditary tendencies, however useful to 

 the organism or important in the evolution of living beings generally, 

 cannot be regarded as the incentive to syngamy at its first appear- 

 ance in the Protista. In other words, amphimixis must be regarded 

 as a secondary consequence, not as a primary cause, of syngamy. 



It is necessary, therefore, to seek some explanation for the 

 first origin of syngamy other than the benefits which it may confer 

 through amphimixis, and it is undoubtedly among Protist organisms 

 that the conditions under which syngamy first arose must be 

 sought. It has been pointed out above that syngamy appears to 

 have a strengthening or recuperating effect upon the cell-organism, 

 and upon such grounds has been founded the theory of " rejuven- 

 escence " (Verjiingung). According to this theory, connected 

 chiefly with the name of Maupas, the cell-protoplasm, after many 

 generations of reproduction by fission, tends to become effete and 

 senile to an ever - increasing degree, a condition which, if not 

 remedied, ends in the death of the organism ; the natural remedy 

 is furnished, however, by the process of syngamy, which has the 

 effect of renewing the " youth " of the cell and starting it upon 

 a fresh series of generations, until senility, once more supervening, 

 necessitates syngamy again. 



The rejuvenescence-theory has been criticized by many critics 

 who have themselves done little more, in some cases, than give a 

 more precise meaning to the terms " youth " and " old age," 

 terms that certainly stand in need of further explanation, since 



