3o8 ANIMAL AGGREGATIONS 



crowding was largely counteracted by the foregoing treatment, and 

 the percentage of males expected from crowded cultures was ma- 

 terially reduced. Considering the wide range of tissue extracts used, 

 the authors appear to be correct in regarding the observed results 

 as due to some generalized factor, such as a change in the bacterial 

 flora, rather than to the action of a specific tissue product. 



The converse experiment, in which the general metabolic rate was 

 experimentally depressed by the use of chloretone or of potassium 

 cyanide in proper concentrations, caused the Moina tested to reach 

 the reproductive age more slowly. The females so retarded produced 

 a much higher percentage of males among their first-broods than did 

 their untreated sisters, which were subjected to similar conditions 

 except for the treatment with depressing agents. 



Stuart and Banta (1929, 193 1) report controlled bacterial studies 

 which indicate that sister Moina females cultured in equal numbers 

 have the sex of their offspring determined in part at least by the 

 numbers of bacteria present. "These females in the highly bacterized 

 bottle produce only female young. Between these two extremes there 

 is a very uniform gradient in male percentage." These authors in- 

 terpret their results as indicating that quantity of food appears to be 

 the determining factor, which causes speculation as to whether the 

 previous analysis of the situation by Banta and Brown is final or 

 whether their findings may not be bound up with as yet unexplored 

 differences in bacterial flora accompanying crowding. The latter is 

 indicated by the conclusion of Stuart and Banta (193 1) that: "Sex 

 of the young produced by experimentally crowded Moina macrocopa 

 mothers can be controlled by the amount of available food (bacteria) 

 present in the medium. The bacteria present in the medium do not 

 appear to influence the control of sex by the adsorption of excretory 

 substances." Here, as so frequently in this survey, we can be certain 

 of the facts of the effects produced by crowding, but, even with a 

 large amount of analytical work, we are not yet sure of the causal 

 factors involved. 



There is no question but that sex is one of the most nearly funda- 

 mental properties of animals. On this account its determination has 

 aroused great interest, and an enormous literature on the subject has 



