NUCLEAR VOLUME AND LIFE-CYCLE OF HYDATINA 315 



might therefore be prevented or modified bj^ unknown agents 

 which would not similarly prevent or modify the changes of 

 reproduction. 



The observations on Hydatina are subject to the same dangers. 

 But owing to the many unrelated ways in which the theory can 

 be tested in this organism, it should be possible to separate 

 fundamental from incidental events. Thus, if the investigation 

 had involved only the yolk gland, and only the changes in the 

 yolk gland during successive generations, it might have been sup- 

 posed that the increase of the nucleoplasma ratio in the yolk 

 gland is the cause of the gradual reduction in the amount of 

 sexual reproduction which has been observed to take place during 

 long-continued parthenogenesis. However, the relative nuclear 

 changes of the yolk gland do not coincide with the other changes 

 in the type of reproduction; for example, not with the changes 

 occurring during the lifetime of the individual, nor with the 

 changes due to different media in which the animals are reared. 

 It must be regarded as accidental, therefore, that the nuclei of 

 the yolk gland increase in volume simultaneously with the diminu- 

 tion of sexual reproduction during a series of generations. 



Notwithstanding the positive nature of the conclusions of 

 Hartmann, these conclusions appear to have been reached partly 

 by inference. The work of Papanicolau was based on too small 

 numbers to be conclusive, and he recognized that it was only sug- 

 gestive. Now that Hydatina is shown to offer no support to the 

 theor}^ that changes of nuclear size determine the changes of 

 reproduction, continued application of the theory to partheno- 

 genetic animals must for the present rest mainly on what are be- 

 lieved to be analogous phenomena in the Protozoa. A note of 

 caution should be sounded with reference to this supposed anal- 

 og3^ Periods of depression in Protozoa, in which nuclear en- 

 largement occurs, are also commonly succeeded or terminated by 

 periods of conjugation. Perhaps it may be assumed that the 

 Cladocera will show similar phenomena, for periods of depression 

 toward the end of a series of parthenogenetic generations have 

 been shown by Papanicolau ('10 a) to be also periods of sexual 

 reproduction. In Hydatina, however, sexual reproduction does 



