W. E. Agar 181 



of their curlier and earlier apjjearance in succeeding generations was 

 observed. 



4. There was a gradual decrease in the number and an increase in 

 the size of the eggs fi-om generation to generation, and also, as regards 

 size, from brood to brood of the same generation. 



He therefore concludes that there is an inherent "sexual cycle" of 

 the orthodox kind in S. vetulus, beginnuig with vigorous parthenogenetic 

 reproduction which gradually gives place to increasing sexuality and 

 degeneracy, ending in total cessation of asexual reproduction. He is, 

 however, prepared to admit that external conditions influence the 

 duration of the cycle to a certain extent. 



The line of S. vetulus whose sexual condition I wish to describe 

 originated fi-om a parthenogenetic female taken from one of the 

 laboratory tanks in September, 1911. I have of course no knowledge 

 of the number of generations which intervened between it and the 

 last ephippial egg. 



Some of the descendants of this individual formed the material on 

 which I did some experiments recently published on the transmission 

 of environmental effects, and also an experiment on parthenogenetic 

 inheritance, not yet published. The method of breeding employed for 

 the great majority of the individuals was that which I described as 

 " normal " or " control " in my former papei'. They were bred in 

 cylindrical glass tubes 10 x 3 cms., which when corked (as they always 

 were) contained about 50 c.c. of water and 15 c.c. of air. With rare 

 exceptions one specimen only was kept in each tube, and when it had 

 produced a brood, this was removed within 36 hours. The culture 

 medium was the water from a tank in which were living a number of 

 Lepidosiren paradoxa, fed daily with Anodonta. The water in this 

 tank was changed weekly (except for that held in a thick layer of mud 

 at the bottom of the tank) and was thick with Infusoria and organic 

 detritus. A jar of water was taken from this tank on alternate days, 

 strained through linen to remove Rotifers, Lynceids, etc. which swarmed 

 in it, and used to renew the water every alternate day in all the breeding 

 tubes. Each Simocephalus was picked out of its tube mth a pipette, 

 the tube emptied and refilled -ndth the new water and the specimen 

 returned. 



The sexual conditions of this line are shown in Table I. 



The sex of a new-born S. vetulus is very difficult to determine, and 

 I did not attempt to do so, as the experiment was not directed towards 

 this question. The sex of every individual which was kept to maturity 



