PERIODICITY IN THE PRODUCTION OF MALES IN 

 HYDATINA SENTA.^ 



a. franklin shull. 



Introduction. 



There is often a well-marked rhythm in the production of males 

 in the rotifer Hydatina senta. Generation after generation may- 

 pass with few or no male-producing females; while later, in a 

 few successive generations, male-producers may be abundant, 

 only to be succeeded by a period in which male-producers are 

 uncommon or wanting. Although this rhythm has not been 

 mentioned by all students of the life cycle of Hydatina, and has 

 been emphasized by few of them, it can hardly have escaped 

 notice by any one who has bred this species for several months. 

 In another genus, Asplanchna, Mitchell (1913) has laid stress 

 upon this rhythmical appearance of males, as a basis for certain 

 theoretical conclusions, and has called attention, by way of 

 generalization, to the same rhythm in Hydatina. 



Regarding the cause of this periodicity, there is not general 

 agreement. Mitchell, writing of Asplanchna but extending his 

 conclusions to rotifers in general, appears at times to regard the 

 rhythm as the effect of an internal factor, and again as due to 

 environmental conditions. He says "this rhythm is not the 

 result of external conditions" but "is not absolutely independent 

 of them. "2 Later he adds that "male production ... is a 

 matter of physiological potential and under the more or less 

 direct control of nutrition" (p. 229). At other places he states 

 that "male production ... is a phenomenon all but wholly 

 under nutritive control ..." (p. 246); "these male and non- 

 male producing strains . . . exist . . . and , . . these strains 

 are also produced by nutritive changes" (p. 247); and "qualita- 

 tive and quantitative changes in nutrition will be found the 

 universal sex-controlling factors in this group" (the rotifers) 



1 Contribution from the Zoological Laboratorj' of the University of Michigan. 

 - Op. cit., p. 228. . 



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