LIFE CYCLE OF SIMOCEPHALfS YETU.US. 65 



justified in concluding that the percentage of males in the genera - 

 tions immediately following the stem mothers is normally low. 

 Such a conclusion would be erroneous, since several experimenters 

 have succeeded in maintaining cultures of Simocephalus and other 

 genera indefinitely in the parthenogenetic phase. In all cases 

 in my experiments where females were isolated and kept in small 

 containers in the laboratory some of their broods were sure to 

 contain sexual forms if the mothers lived to produce a reasonable 

 number. In the experiment under discussion the sexual forms 

 did not increase up to the sixth brood, after which so large a 

 proportion of stem mothers died that it seemed best to close the 

 experiment. However at this point experiment 5 is comple- 

 mentary since it is concerned with a large number of females 

 which are of varying degrees of remoteness from stem mothers 

 and we find them producing only about five per cent, of sexual 

 forms. It seems quite clear that the remoteness of the generation 

 from the stem mother bears no definite relation to the numerical 

 ratio of male to female offspring or of sexual to asexual females. 



As to the degree of sexuality of the sexual females concerned 

 in this experiment I find no evidence that it is different from that 

 observed in the former experiment. The number of ephippia 

 produced can doubtless be taken as a crude index to the degree of 

 sexuality of a female, since, as shown elsewhere, the number pro- 

 duced is dependent upon factors inherent in the female, fertiliza- 

 tion of the ephippial egg having no relation whatever to the 

 continuance of the sexual state. The number of ephippia cast 

 by each sexual female in this experiment averages slightly less 

 than two. I have found very few females which produce as 

 many as five ephippia. Only three instances are recorded in all 

 of my experiments. The production of three is common. Here 

 as in all other cases observed, the sexual individuals passed the 

 sexual state by the time they \\ ere about half to two thirds grown, 

 and never returned to it once they had become asexual. The 

 only observer so far as I am aware recording the contrary for 

 Simocephalus vetulus is Issakowitsch (1908). Moreover, ephippia 

 which were barely noticeable would sometimes appear and be 

 cast, asexuality coming on at once. Other individuals would 

 develop their ephippia to half or two thirds the normal size and 



