PARTHENOGENETIC AND SEXUAL REPRODUC- 

 TION IN SIMOCEPHALUS VETULUS AND 

 OTHER CLADOCERA. 



By W. E. AGAR, 



University of Glasgow. 



In 1879 Weismann formulated the theory that in the Cladocera "der 

 Eintritt der Geschlechtsperiode wird iiberhaupt nicht durch direct 

 wirkende aussere Momente veranlasst, sondern lediglich durch innere 

 in der Natur des Organismus begriindete." There can be no doubt that 

 this total denial of the influence of environment can no longer be main- 

 tained. It is as certain as experiment can ever make it, that under 

 some conditions parthenogenetic reproduction will proceed longer than 

 under others, and that to some degree the appearance of sexual forms 

 can be controlled by controlling the environment. The latest and 

 probably the most conclusive evidence of this is given by Grosvenor 

 and Smith for Moina. It is true that many experimenters have failed 

 to find any evidence of the direct effect of environment, but this 

 negative evidence cannot be put against the quite satisfactory positive 

 results of other workers. On the other hand, even those who admit the 

 influence of environment do not give up the idea of an underlying 

 " cycle " as well (Issakowitsch, Papanicolau, Woltereck, R. Hertwig). 



The object of this note is to present some new evidence and point 

 out some general indications that the change from parthenogenetic to 

 sexual reproduction, and the degeneration which so often accompanies it 

 under experimental conditions, is determined by environment only, and 

 that the number of preceding parthenogenetic generations or the lapse 

 of time since the last sexual act are not, as such, relevant to the matter 

 at all. 



My own experiments have dealt chiefly with Simocephalus vetulus, 

 and were not undertaken in relation to the reproductive cycle problem, 



