REPRODUCTION 33 



occurs. While such a process has not been observed in many 

 protozoans, it presumably occurs in all under certain conditions. 

 The analogy between a protozoan life cycle and a metazoan 

 life cycle has become understood only in recent years. As a 

 result of the painstaking experiments of Calkins and other pro- 

 tozoologists, it is now usual to compare the entire life cycle of a 

 protozoan animal from one sexual reproduction to the next, 

 including all the intervening asexual generations, resulting per- 

 haps in millions of individuals, with the life cycle of a single 

 metazoan. According to this view the asexual reproduction, 

 as remarked above, is comparable with the multiplication of 

 cells in a metazoan body, except that, instead of all the cells 

 resulting from such multiplication remaining together and be- 

 coming specialized for particular functions, they separate and 

 live as independent individuals. Just as the cells of a meta- 

 zoan body grow old after a variable length of time and lose their 

 youthful vitality and reproductive power, so the protozoan 

 cells, after a variable number of multiplications, gradually lose 

 their vitality and reproductive power. In the metazoan certain 

 cells have the power of renewing their waning vitality by union 

 with a cell of the opposite sex (sexual reproduction), thus be- 

 ginning the cycle again. In the protozoan the sexual phenomena 

 which have been observed are believed to have the same signifi- 

 cance, and there is evidence that at least in some Protozoa the 

 sexual power may be confined to certain individuals which would 

 then be comparable with the sex cells of the metazoans. Calkins' 

 experiments led him to believe that in JParavicecium, a common 

 ciliated protozoan on which he experimented particularly, old 

 age and death were inevitable after a variable number of asexual 

 generations without sexual reproduction. It has recently been 

 shown, however, that when conditions of life are perfect, Para- 

 mecium may continue to multiply asexually for an indefinite 

 time. Periodically, however, a complete reorganization of the 

 cells occurs which apparently has an effect similar to that pro- 

 duced by sexual reproduction, the animals having renewed vi- 

 tality for many generations. This remarkable process, named 

 "endomixis," is strikingly analogous to parthenogenesis (de- 

 velopment of unfertilized eggs) in higher animals. Another 

 analogy is that under unfavorable or adverse conditions sexual 

 reproduction replaces endomixis, just as in such animals as 



