"STRAINS" ix HYDATINA SIM \. 207 



generation of each race were isolated at the same time and each 

 female placed in a Syracuse watch glass and allowed to mature 

 and to product- daughter females. Then this pn>cc was re- 

 peated tor .^45 generations. All the females at each isolation 

 urn- placed in the same quantity of tap water to which \\a- 

 added the -ame amount of food culture that was taken from one 

 food jar. The watch glasses in which the rotifers lived al\\a\- 

 \\en- in thn-f -tacks side by side at room temperature. IVac- 

 ticallv all external influences were as identical as it was po ible 

 to make them. 



Tin- del tiled observations are given in Table III. in parallel 

 column- and the -ummary is given in Table IV. 



At ill' <nd of Table I., races A and B, which up t<> tin- time 

 were l< d on \arious protozoa cultures, were practically identical 

 in re-ard to the percentages of male-producing females in each 

 . but at the beginning of Tables III. and IV. when the t \\ o 

 races \\ere -ubjected to. uncooked concentrated food culture 

 media a de< ided change occurred. Race .1 retained and e\en 

 ede. I its former rate of production of male-producing female-, 

 but in ra.e li the rate was very perceptibly lowered. K 

 louered -liuhilv its rate of male-producing females. Thi- 

 runvd during the first 50 generations. From the 57th to the 

 ^I5ih generation in races A and 6' and to the end of race />'. t lu- 

 ll ;o -iii -ration, concentrated cooked food media \\a- u-cd ami 

 cau-eil a decided lowering of the production of male-producing 

 female- in all race-. In race t' this was reduced to zero, in i 

 />' to less than I per cent., and in race -1 to about 3.5 per cent. 



Tin- confirm- the earlier results in showing that it i- po--ible 

 1>\ external conditions to repress entirely the production of male- 

 pr.'du. in. females in some races of this rotifer for a long period 

 of time. In race (' the male-producing female- were repre ed 

 for Hi-rations and then reappeared when the food media 



\\a- made too dilute accidentally. 



If the-e three races were exactly alike in their power to produce 

 male-producin- females and all were subjected to the -ame ex- 

 ternal conditions they ought to produce -uch male-pro<hn in- 

 female- at the same rate. Howe\ er, as the <ibo\e ob-er\ ation- 

 -ho\\ that the rates of production of male-la\ in- leniale- vary 



