The Life-Cycle of Cladocera. 419 



to ensure that there is always an excess of nourishment, because the culture 

 consists only of the Alga and not of a mixed assemblage of bacteria, some of 

 which may be useless as nourishment. 



The scheme of the experiments is as follows : — In each generation a certain 

 number of the individuals are isolated soon after birth in separate glasses ; 

 some of these are placed in an incubator at 27° C, others are stood in a water 

 tank with circulating water at 10-17° C. Other individuals are kept crowded 

 together in the same glass in numbers of 10-15, and of these crowded glasses 

 some are again placed in the incubator and others in the circulating water at 

 10-17° C. Thus in each generation we have individuals subjected to four 

 different conditions :— (1) Isolated at 27° C. ; (2) crowded at 27° C. ; 

 (3) isolated at 10-17° C. ; (4) crowded at 10-17° C. All are supplied with 

 excess of Protococcus. 



In Table I is given the result of breeding under these various conditions 

 for eleven successive generations. This Table does not give the numbers of 

 ephippial females which appeared among the parents, but it may be stated 

 that ephippial or sexual females only appeared among the parents kept 

 crowded at 10-17° C, which also gave a high percentage of males. 



It will be noted that in this Table, besides the numbers of male and female 

 offspring produced in each generation, a column is devoted to the number of 

 parents, whether isolated or crowded, which were used for breeding. This 

 factor, viz., the number of parents used, is one that must not be lost sight of, 

 since, in order to prove that the production of males and sexual females is 

 not simply a question of chance, it is clearly necessary to use a sufficient 

 number of parents in each generation and under each condition to ensure 

 that the effects of chance are ruled out. In as many cases as possible four 

 broods were taken from each female. It was not found that there was any 

 tendency for later broods to produce more sexual forms than early broods. 



By consulting Table I it will be seen that neither in the isolated nor 

 crowded individuals at 27° C. did any sexual forms appear throughout the 

 eleven generations. Adding the totals of the isolated and crowded at 27° C. 

 together we have, 90 + 150 = 240 parents gave 1188 + 643 = L831 partheno- 

 genetic females and no males. 



Eighty-six individuals isolated at 10-17° 0. gave 1860 parthenogenetic 

 females and 56 males, or about 3 per cent, males, while 420 individuals 

 crowded at 10-17° C. gave 3564 parthenogenetic females and 256 males, or 

 about 7 per cent, males. 



These results are in agreement with our previous experiments with Moina, 

 showing that the effect of isolation and high temperature is to suppress the 

 production of the sexual forms (5). 



