LIFE CYCLE OK SIMOCEPHALUS VETULUS. 69 



month in the year from shallow ponds near (iary, Ind. The 

 offspring obtained in the winter months were asexual females. 

 While no sexual females and males were obtained from eggs and 

 embryos borne by females at the time of collecting from under 

 the ice, the offspring thus secured were not sufficiently numerous 

 to justify any statement as to the production of males and sexual 

 females at this season. I am uncertain as to just how far these 

 experimental results hold true in nature, but there seems to be 

 no reason for considering temperature a vital factor in relation 

 to the sex cycle of Simocephalus vetulus. As mentioned else- 

 where, Grosvenor and Smith (1913) completely inhibited sexual 

 forms in Moina rectirostris at a temperature of 28 C., but since 

 they could not decide as to what factor their success was due it is 

 probably safe to assume that they succeeded in spite of the high 

 temperature, not because of it. 



Experiment 5. The following experiment was performed to 

 discover what kinds of offspring are produced under natural 

 conditions. It involves 51 females collected at various times 

 from April 20 to May 25, in the vicinity of Northfield, Minn., 

 from three widely separated permanent freshwater ponds. The 

 sizes of the females ranged from small (hence young) to very 

 large (hence old) individuals. The largest female among the 

 lot was slightly over 4 mm. from the anterior margin of the head 

 to the posterior end of the carapace and 2\ mm. in vertical 

 measurement. There is no doubt that the age and size of 

 Simocephalus correspond very closely. Though I know the 

 pedigree and brood records of none of these 51 females it may 

 be assumed on the basis of size that some, such as the smallest, 

 had produced very few broods, while others, the larger ones r 

 had produced very many, probably 15 or 20. Although the 

 individuals were not isolated there was no crowding. They 

 were placed in two-quart fruit jars, no jar containing more than 

 six individuals. Jars containing several specimens were kept 

 full. They were placed outside the laboratory windows where 

 the sun would not strike them, being thus subject to the ever 

 varying temperatures. Only water brought from the ponds 

 where the females were collected was used. This was strained 

 through a fine silk cloth which removed all metazoa, but allowed 



