The Life-Cycle of Cladocera. 



421 



The result which is given in Table II shows that during eight successive 

 generations 117 isolated parents at 27° C. produced 2564 parthenogenetic 

 females, no males, and in no case became ephippial, while 129 crowded 

 parents became ephippial in 17 cases and produced 1147 parthenogenetic 

 females and 26 males. 



Since nearly equal numbers of parents were used and more offspring were 

 produced from the isolated parents than from the crowded, it is impossible to 

 ascribe the production of sexual forms by the crowded individuals, and their 

 entire absence in the case of the isolated parents, to chance. 



If we add together the results for the isolated at 27° C. and for the crowded 

 at 10-17° C. in the two experiments given in Tables I and II, we see that in 

 nineteen generations 207 isolated parents at 27° C. gave 3752 parthenogenetic 

 females and no males, while 549 crowded parents at 10-17° C. gave 4711 

 parthenogenetic females and 282 males, or about 6 per cent, males. It is 

 also to be observed that, while no ephippial females appeared among the 

 isolated parents at 27° C, about 10 per cent, of the crowded parents at 

 10-17° C. became ephippial. 



Table II. 





Isolated at 27° C. 



Crowded at 10-17° C. 



Genera- 







Number 







Number 



tion. 



Number 



Parents 



of offspring. 



Number 



Parents 



of offspring. 





of 



becoming 







of 



becoming 









parents. 



ephippial. 







parents. 



ephippial. 













Female. 



Male. 







Female. 



Male. 



35 



8 







132 







10 



2 



63 



10 



36 



16 







334 







20 



1 



117 







37 



19 







414 







20 







222 







38 



16 







350 







10 



1 



111 







39 



13 







397 







20 



4 



116 







40 



. 9 







i53 







10 







264 







41 



16 







644 







10 



2 



59 



1 



42 



20 







140 



. 



29 



7 



195 



15 



Totals 



117 







2564 







129 



17 



1147 



26 



The general result of the above records is to show that in D. pulcx, 

 as in M. rectirostris, it is possible to inhibit entirely the appearance of 

 males and sexual females by isolating the parents soon after birth and keep- 

 ing them at a temperature of 27° C. But if we look into the numbers given 

 for each generation in Tables I and II, we find that the converse of the above 

 statement does not hold good, i.e., it is not the case that crowding at 

 10-17° C. always results in the production of the sexual forms. Thus, to 



