LIFE CYCLE OF SIMOCEPHALUS VETULUS. 89 



several genera of Cladocerans for an indefinite number of genera- 

 tions. Their conclusions are more fully discussed under the head 

 of literature review. 



In my earlier experiments there are numerous instances of 

 the production of all three kinds of individuals, namely, males, 

 sexual females and asexual females, by the same mother. Less 

 common is the production of only two kinds of offspring during 

 the life time of a female. In these experiments I have no record 

 of a female producing only one kind of offspring except those 

 whose progeny are very limited because of the early death of 

 the mother, and in recent experiments, those which produce 

 only asexual females, this last being of much importance. Rarest 

 of all yet quite often met with is the production of all three kinds 

 of offspring in the same brood. Female number 45 of isolation 

 experiment 2 has one such brood containing only three embryos. 

 Broods of this type are usually larger. The asexual females have 

 nearly always outnumbered the sexual females and males in 

 mixed broods, as they usually do in the sum of the broods of a 

 given female. 



Similar proportions of the kinds of offspring may be secured 

 under a great variety of kinds of environment. In the most 

 highly sexual cultures many pure asexual females are always 

 present, and the sexual females remain so only for a brief period 

 in early life. While these facts do not prove that the environ- 

 ment has nothing to do with the ratio of kinds of offspring, 

 they would seem to indicate that it is not the determinative 

 factor in the nexus of causes that are responsible for the expression 

 of the species in all of its forms, regardless of internal factors. 



VIII. SUMMARY. 

 I. Life History. 



i. The offspring of any female of Simocephalus vetulus in the 

 asexual phase may consist of any one or all three of the following 

 kinds of individuals: 



(a) Sexual females, which produce a series of from one to 

 seldom more than six ephippial eggs early in life, then become 

 parthenogenetic and so remain, being then indistinguishable 

 from other parthenogenetic females. The final ephippium is 



