312 Aaron Franklni Shiill 



important problem in Hydatina, therefore, is to determine what 

 condit ons, whether external or internal, bring about the change 

 from the parthenogenetic to the sexual mode of reproduction, 

 and so affect the proportion of male-producers. With a view 

 to testing anew the influence of the various agents which have 

 been supposed to alter this proportion, the experiments described 

 in this paper were performed. The results of the experiments 

 do not support the view that any of the proposed agents have a 

 direct influence on the proportion of male-producers; but they 

 do give evidence of another factor, not hitherto suggested, a 

 factor which not only accounts for the results of the present experi- 

 ments, but aff^ords a simple probable explanation of the results 

 upon which the previous contradictory conclusions were based. 

 The results of the experiments indicate that the presence of cer- 

 tam substances dissolved in the water in which the rotifers are 

 reared may exert a potent influence on the proportion of male- 

 producers; and they make it probable that the agents formerly 

 thought to .exert such an influence either have no influence at 

 all, or exert it only indirectly by first aff"ect'ng the dissolved sub- 

 stance 5. The previous contradictory conclusions may thus be 

 brought under a common point of view. The above conclusion 

 regarding dissolved substances was reached only after a number of 

 tests had been made of the factors previously bel eved to influence 

 the proportion of male-producers; in the account given in th's 

 paper, the experiments are described approximately in the order 

 in which they were performed. 



The 1 fe cycle of Hydatina senta is as follows: From the ferti- 

 lized or resting egg there hatches invariably a female. This fe- 

 male may, under favorable circumstances, produce parthenogenet- 

 ically 40 to 50 off'spring, all of the same sex, which so far as I have 

 observed is always the female. These females may in turn pro- 

 duce 40 to 50 young, all the off'spring of one parent being of the 

 same sex; but while some of the females produce females, others 

 may produce males. Thus the females may be spoken of as male- 

 producers or female-producers. The number of male-producers 

 in. a family varies within wide limits; it may be zero, or it may 

 be ICO per cent. When males have appeared in a colony, resting 



