86 \\VM.\\ REED GREEN. 



females in the first generation (10 X 6 X 1.3 = = 78) ; and suppos- 

 ing there were in all only 5 females to a brood, (see table of same 

 experiment), we should have 300 females to mother the F 2 genera- 

 tion, which should yield 23,400 ephippial females, out of a total 

 of 90,000. Of course in practice, laboratory cultures are over- 

 stocked long before all of the second generation has appeared, 

 and in fact reproduction has practically ceased by the time 100 

 to 200 ephippia have appeared. The accumulation of the ephip- 

 pia makes the culture seem to be in the sexual phase. The 

 variation in the number of kinds of offspring produced may de- 

 pend to some extent upon the extreme prolificacy and conse- 

 quent mortality in laboratory cultures, since the assumption of 

 ever so slight a difference in the susceptibility of embryos to 

 the great variety of adverse conditions met with in laboratory 

 cultures, would be sufficient to account for much of the seeming 

 variation. One may sometimes find several hundred ephippia 

 in a laboratory culture in which there are very few males. In 

 one such instance I collected 200 ephippia, only 2 of them con- 

 taining eggs. Four explanations are possible: they may have 

 hatched, which is very unlikely, there may have been too few 

 males to fertilize them, mating may have been inhibited by some 

 unfavorable cultural condition, or, there may have been abnormal 

 pairing such as I have described in the section on pairing, in 

 which case the spermatozoa not being retained, the eggs would 

 degenerate. I have had a few small cultures in which for a short 

 time males were very numerous and sexual lemales almost ab- 

 sent, but such instances are rare. 



The following is typical of my experience with Simocephalus. 

 I isolated a female on July i, 1913, and kept her in lake water 

 from lake Michigan. This was changed daily. She had nothing 

 to eat except what she could get from the lake water. This 

 seemed barely sufficient since reproduction proceeded very slowly. 

 On August i 1 placed o of her descendants in a rich algae culture. 

 Here they multiplied very rapidly. By August 13 there- were 50 

 ephippial eggs floating Oil t he Mirlace, and males were also present 

 in the offspring. This culture continued to produce asexual 

 mm- lor some months, \\hen the alga 1 gradually died oni m- 

 in the culture became less numerous, reproduction 



