CROWDING AND SEX DETERMINATION 305 



1923 and not yet finished, have found that crowding of the mothers 

 in 2 species of Daphnia, 3 of Smioccphalus, 3 of Moina, and i of 

 Ceriodaphnia causes a large number of males to appear. Agar has 

 recently informed me personally that he has confirmed these results 

 "to the hilt," although his pubhshed work (1914) showed that males 

 will appear without crowding and with frequent changes of medium. 

 Shelley (1929), working with another species of Daphnia (D. magna) 

 in this laboratory, has obtained results similar to those reported by 

 Banta and Brown. There can be no doubt of the fact. 



The later work of Banta and Brown gives an opportunity to show 

 the extent of the changes produced in the sex of the offspring. In 

 all they have performed some 2,900 experiments dealing with the 

 effect of crowding on male production in cladocerans. One lot of 

 109 uncrowded mothers which produced 1,954 young, averaged 

 only 0.3 per cent males; 5 males were produced by one mother, and 

 a single male by another. On the other hand, t,;^ moderately crowd- 

 ed culture bottles which contained from 7 to 14 females, usually 

 10 per culture bottle, gave an average male production of 41.6 per 

 cent out of 3,638 young whose sex was determined. Six of these 

 bottles failed to produce males; 2 produced 100 per cent males. 

 In heavily crowded bottles, with 15 to 24 mothers to the bottle, 

 from 2,240 young whose sex was determined, 62 per cent were males. 

 One bottle failed to produce males, and 4 produced 100 per cent 

 males. They regard these as typical results. It will be noted that 

 the effects of crowding are not uniform and that quantitative male 

 production does not occur in all the bottles of a densely crowded 

 series. 



We find that Banta and Brown conclude from their experiments, 

 as Grosvenor and Smith did earher, that the chief factor in stimu- 

 lating male production in Moina is the accumulation of excretory 

 products. Further, they present evidence which shows that this 

 effect of crowding can be produced by the presence of other genera 

 of Cladocera or by other aquatic animals, such as Planaria, Asellus, 

 Physa, insect larvae, small fish, or frog tadpoles; monotypic excre- 

 tory products are unnecessary. Here, in the heterotypically pro- 

 duced adverse conditions, there is a lack of species specificity of 



