FOOD AND OXYGEN IN CONTROLLING SEX 135 



under conditions as nearly identical as is possible to make them 

 produced no male-producing daughters. As has been pointed 

 out before the food content, judged by the number of contained 

 bacteria of the different watch glasses is scarcely ever identical 

 at any time during the experiment. Oxygen usually favors an 

 increase in the number of bacteria but if too many bacteria are 

 produced they or their products interfere with the normal 

 metabolism and reproduction of the rotifers. If, on the con- 

 trary, a sufficient number of bacteria are not maintained in the 

 culture waters the rotifers are deprived of more or less of food 

 supply. It is quite possible that in these conglomerate mixtures 

 of bacteria in the zooglea all kinds of bacteria are not useful 

 directly or indirectly as food for the rotifers but that only cer- 

 tain species are useful. As there are so many millions of indi- 

 viduals in a watch glass at the beginning of the experiment 

 it is very likely a matter of chance which species will survive 

 and maintain itself or will even increase its own numbers. As a 

 matter of chance some of the watch glass cultures in both the 

 air and the oxygen experiments develop those species of bacteria 

 that cause favorable food conditions for the rotifers to produce 

 male-producing daughters while other cultures in the same series 

 of experiments do not develop those bacteria which cause favor- 

 able food conditions for the rotifers and consequently only 

 female-producing daughters are produced. 



A fine illustration showing the relative effect of oxygen and 

 food in causing male-producing females to be produced is seen 

 in experiment I of table 6. The rate of metabolism and repro- 

 duction was easily ascertained by noting the increase or de- 

 crease in the number of eggs found attached to each female- 

 producing female. When there was an abundance of food each 

 female might carry five to eight eggs but when there was only a 

 moderate or scanty amount of food a female-producing female 

 would only carry one to two eggs or if food was too scanty she 

 would carry no eggs at all. When oxygen was allowed to pass 

 through the jar of culture water containing miscellaneous pro- 

 tozoa and the rotifers for twenty-four hours the females soon 

 began to produce more eggs. Before the oxygen was allowed to 



