W. E. Agar 315 



namely, excessive production of sexual eggs, and these three features 

 continued to manifest themselves throughout the ten generations for 

 which the clone has been bred. Some females showed indeed a much 

 greater degree of sterility than the original hybrid. One $ in genera- 

 tion 3, for example, started by producing two or three sets of sexual 

 eggs, then seven successive batches of parthenogenetic eggs broke up 

 in the brood pouch without undergoing any visible development. The 

 eighth parthenogenetic brood consisted of three eggs, of which two failed 

 to develop and one died as a young embryo. Two more sets of par- 

 thenogenetic eggs were then laid, all of which disintegrated without 

 development. The parent then died, having only produced one dead 

 embryo out of ten batches of parthenogenetic eggs. 



Several other females of this clone laid batches of parthenogenetic 

 eggs regularly, but never produced any living young. 



These are of course extreme instances, but a high degree of partial 

 sterility was general in the females of this hybrid clone. It is difficult 

 to give figures of the proportion of eggs which failed to develop, since 

 the majority of them break up very soon after laying, and unless one 

 happens to observe the $ immediately after oviposition, nothing remains 

 of them but loose grains of yolk. There is no doubt however that con- 

 siderably more than half the parthenogenetic eggs produced by this 

 clone have failed to develop. 



A certain amount of pre-natal mortality has been evident even in 

 my pure bred Daphnia clones, especially of D. pulex, but the rate does 

 not approach that of the hybrid clone. 



Records were taken of the sexes of the living broods produced by 

 eighteen of the daughters of the original hybrid, with the following 

 results : 



I have no exact records of the properties of the different types of 

 broods produced by the two parental clones, but from a long experience 

 of them I should estimate that D. obtusa, clone H, produces not more 

 than 3 °/^ of sexual broods and 1 °/^ of male broods, and D. pulex, 

 clone RR, not more than 10 °/^ sexual broods and 5 "/„ male broods. 

 These figures apply of course to the conditions under which they and 

 the hybrid clone were bred. 



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