154 PROTOZOOLOGY 



aurelia between May 1, 1907 and May 1, 1915, did not manifest 

 any decrease in vitality after eight years of asexual r(>])roduction, 

 demonstrates beyond doubt that the sexual reproduction in the 

 form of conjugation is not necessary for the well-being of Para- 

 mecium aurelia under favorable environmental conditions. On the 

 other hand, there is a large body of evidence to sup])ort the view 

 expressed first by Maupas to the effect that the conjugation cor- 

 rects an inherent tendency toward senescence under unfavorable 

 conditions. Recently Sonneborn and Lynch (1932) demonstrated 

 by using different clones of P. aurelia that the effects of conjuga- 

 tion are diverse and characteristic of different races: 1) the con- 

 jugation increases fission rate in some clones, decreases the rate 

 in others; 2) it increases variation in some clones, but not in 

 others; and 3) it increases mortality in some clones but not in 

 others. Sonneborn (1937) continuing controlled observations on 

 this ciliate, discovered that in certain races there are two classes 

 of individuals with respect to sexual differentiation and that the 

 members of different classes conjugate, while the members of each 

 class do not. He further found that the individuals produced by 

 binary fission from a single individual belong all to the sex re- 

 action type to which the original individual belonged, and that in 

 conjugation in which two sex reaction types participate, the four 

 sets of progeny consist of the two types in chance combination, 

 the ratio being identical with those for inheritance in higher 

 organisms. Jennings (1938) found further four sex raction types in 

 P. bursaria, in which the type behavior toward conjugation was 

 exactly like that of the two types found in P. aurelia. 



Automixis. In certain Protozoa, the fusion occurs between two 

 nuclei which originate in a single nucleus of an individual. This 

 process has been called automixis by Hartmann, in contrast to 

 the amphimixis (Weismann) which is the complete fusion of two 

 nuclei originating in two individuals, as was discussed in the 

 preceding pages. If the two nuclei which undergo a complete 

 fusion are present in a single cell, the process is called autogamy, 

 but, if they are in two different cells, then paedogamy. The 

 autogamy is of common occurrence in the myxosporidian spores. 

 The young sporoplasm contains two nuclei which fuse together 

 prior to or during the process of germination in the ahmentary 

 canal of a specific host fish, as for example in Sphaeromyxa 

 sahrazesi (Figs. 209; 210) and Myxosoma catostomi (Fig. 208). In 



