DEPARTMENT OF GENETICS. 



117 



the latter ratio can be regularly produced, sex-ratio is said to be 

 controlled. Control of the sex-ratio offers an interesting field for 

 experimental research, the importance of whose applications to man 

 are obvious. 



Cladocera.— In the Year Books for 1917 (p. 119) and 1918 (p. 107) 

 have been described Dr. Banta's attempts to control the sex-ratios in 

 these small crustaceans. This year further progress has been made and 

 all species or strains which were adequately tested yielded greater or 

 less evidence of sex-control, or at least an influencing of the sex-ratio. 

 While in 1917 the addition of acid yielded "all male" broods, this year 

 the method used was that of crowding the mothers by placing 10 or 

 more in a bottle containing approximately 75 c. c. of culture water. 

 Such crowded mothers produced from a small percentage to over 90 per 

 cent males in individual bottles. The species used were Daphnia 

 pulex, Simocephalus exspinosus, and Moina inacrocopa. Oidinarily in 

 nature they produce few or no males. 



Table 4. — Summarized data of some Cladocera sex-control experiments. Number of off- 

 spring of each sex, and percentage of male offspring. 



From table 4 it appears plain that the proportion of males increased 

 enormously in the bottles with 10 or 20 females, as compared with those 

 containing only 2 females. The most probable explanation seems to 

 be that crowding, probably by influencing the quality of the water, 

 induces male production. Dr. Banta reports further: 



"The different species and lines vary in their susceptibility to the sex- 

 control measures. The above are representative data of the sex-control 

 experiments. Moina macrocopa is perhaps the most responsive of the species 

 used, though the two experiments with S. exspinosus gave sUghtty higher 

 male percentages. Daphnia pulex is the least responsive species so far 

 sufficiently tested and different lines of this species vary in their suscepti- 

 bility to the influence of crowding. 



"One line of Daphnia pulex, when subjected to crowding, has so far pro- 

 duced sexual eggs (ephippa) rather than males. This is again suggestive of 

 differences in susceptibility of the different lines to the same influence. The 

 influence calling forth the production of males is obviously related to the 

 production of ephippial eggs, else the whole mechanism of sexual repro- 



