Life Cycle of Hydatina Senta 345 



Male-producers are not more abundant at one end of the family 

 than at the other, regardless of whether the family be large or 

 small. 



One female may lay both fertilized eggs and male eggs. 



DISCUSSION 



The first stage in the solution of the problem undertaken in 

 these studies seems to have been reached in Experiments IX and 

 X. The results of these experiments indicate that in Hydatina 

 senta the proportion of male-producers is reduced by certain dis- 

 solved substances in the water in which the rotifers are reared. 

 The full force of this discovery is not at first apparent, and the 

 explanation of the two experiments in question is not the measure 

 of its importance. Not only may nearly all the results of experi- 

 ments dealing with the proportion of male-producers, which are 

 described in this paper, be accounted for by this new factor; but 

 practically all the work of previous investigators, which led to 

 contradictory conclusions, may be simply explained by the same 

 means. It is thus possible, w'thout wholly rejecting the conclu- 

 sions of earlier workers, to bring their apparently discordant re- 

 sults under a common point of view. 



In starvation experiments, it is not practicable to use a smaller 

 quantity of protozoan food, without at the same time introducing 

 a smaller quantity of the substances dissolved in the food culture. 

 The results attributed to starvation may in reality be dependent 

 on the reduced quantity of such substances. The difference in 

 the proportion of male-producers apparently resulting from 

 starvation is of the same sign as should result (according to Ex- 

 periments IX and X) from the influence of these dissolved sub- 

 stances; and as the difi^erences obtained in the starvation experi- 

 ments (III and IV) are not greater than may easily be explained 

 by this factor alone, it may be doubted whether starvation per 

 se has any effect whatever. Nussbaum's conclusion that starved 

 rotifers yielded more male-producers than well-fed ones, seems at 

 first sight to be justified; but the effects which he noted were 

 probably due, not to the scarcity of protozoan food, but to less 

 concentration of certain substances in the water. 



