84 \\VM.\X REED GREEN. 



5. What relation has sexuality to the duration and senescence 

 of the laboratory cultures? 



The fact that my cultures which have produced sexual females 

 and males have always passed sooner or later into a more or 

 less non-productive phase and did not die out provided that con- 

 ditions were not so severe as to terminate them, is well deserving 

 of consideration, in view of the fact that one so often reads in 

 Daphnian literature of cultures becoming sexual and being there- 

 by terminated. During the five years I have worked on Simoce- 

 phalus I have run scores of cultures and I have never had an in- 

 stance of a pure sexual culture except those set up by selecting 

 males and sexual females from other cultures. At the most such 

 cultures remain purely sexual only a few days, when with the 

 passing of some of the sexual females into the asexual phase, 

 broods invariably appear containing asexual females. I have no 

 record of a large pure brood of sexual females, and no record of 

 a female which has consistently produced pure sexual small 

 broods. Broods containing sexual females may be produced by 

 any female no matter what her pedigree has been. Naturally 

 if a large percentage of the females in a culture are passing through 

 the sexual phase, so much of their immediate productivity is 

 sacrificed, but since the number of ephippial eggs produced rarely 

 exceeds 4, and is usually only I or 2, and always being produced 

 within the first 10 to 25 days of their lives, every sexual female 

 devotes the most of her life to asexual reproduction. Old cul- 

 tures, in which there has been much accumulation of excreta, 

 run down, the size of the broods decreases to from i to 5, many 

 individuals die before maturity, and the culture dies out finally; 

 but so far as I am able to see, this neither causes nor results from 

 the production of ephippial eggs. 



It is of interest to consider what happens in laboratory cul- 

 tures which are set up and allowed to run their natural course un- 

 hindered. With each female producing a brood of from 3 to 35 

 every 40 or 50 hours, as is always the case in a culture containing 

 an abundance of fresh green alga.' and no accumulation of excreta, 

 any culture, even though started \\itli a single individual, will 

 1 overstocked in a few weeks, and the increase in numbers 

 'op, the development of eggs in the ovaries and the 



