3o6 ANIMAL AGGREGATIONS 



effects, just as we have already seen a lack of species specificity in 

 group protection from toxic solutions or hypotonic sea-water. These 

 excretory products efifective in male production are apparently not 

 volatile, but they are unstable compounds readily made non-effec- 

 tive by a variety of treatments. Standing for a short time, or treat- 

 ment with NaOH or H2SO4, or a greater dilution of the medium, is 

 effective in causing a lack of potency for male production. 



The factors causing production of sexual eggs are apparently 

 different from those causing males to appear. Sexual eggs are 

 produced only rarely in the crowded bottles under the conditions 

 used to produce males. In Moina macrocopa they are produced by 

 rearing females in clear pond water, or in very dilute food, or in old 

 culture media. Banta and Brown believe their experience shows 

 that scarcity of food controls the production of the sexual egg, as 

 the accumulation of excretory products as a result of crowding 

 controls the appearance of males. 



Further studies have indicated that the time at which the sex 

 of the forthcoming young of this cladoceran is determined Hes about 

 4 hours before the parthenogenetic eggs are laid, when the tempera- 

 ture stands at 20° C. (Banta and Brown, 19296). Observation that 

 there was some relationship between the time of the release of a 

 female's first brood and the sex of the young led to definitive exper- 

 iments concerning the relationship between the rate of the mother's 

 development and the sex of her offspring. The evidence from these 

 experiments is summarized in part in Figure 30, which indicates 

 that the production of males is closely associated with a reduction 

 in the rate of development of the females producing them. Appar- 

 ently this is due to the accumulation of excretory products in the 

 crowded bottles, since the reduction in time of production of the 

 first clutch of young is proportional to the degree of crowding, and, 

 further, the percentage of males produced is proportional to both. 



Tests were made (Banta and Brown, igigd) to find whether 

 experimentally changing the rate of development would affect the 

 sex ratios of the produced young. Treatment with small dosages of 

 ethyl alcohol and with filtrates from dried adrenal cortex, thyroid, 

 thymus, and muscle tissue serves to increase the rate of development 



