2 David Day Whitney 



Both female-laying females and male-laying females can be 

 impregnated by males, but on the former, impregnation is sup- 

 posed to have no effect. If the male-laying females are impreg- 

 nated by the male in the first few hours after they leave the egg, 

 such females produce fertilized eggs instead of parthenogenetic 

 male eggs, thus shov^ing that male-laying females can develop into 

 sexual females that lay fertilized eggs. 



The female-laying female can produce a family of daughter- 

 females, some of which may lay female eggs and others may lay 

 male eggs. 



With the view of finding out the ratio in which these two classes 

 of daughter-females are produced under various conditions I have 

 carried out the experiments to be described. 



Maupas found that a temperature of 26° to 28° C. would pro- 

 duce as high as 95 per cent of male-laying females while a tem- 

 perature of about 14° C. would produce as low as 5 per cent of 

 male-laying females. 



Nussbaum, on the contrary, came to the conclusion that nutri- 

 tion and not temperature is the sex controlling factor. He found 

 that by starving the young females for the first few hours after they 

 emerge from the egg they would produce a high percentage of 

 males, but if they were fed at the time they leave the egg they pro- 

 duce a high percentage of females. 



Punnett has carried out a few experiments along the lines laid 

 down by Maupas and Nussbaum and finds that neither tempera- 

 ture nor nutrition is influential in determining the sex. He finds, 

 on the contrary, that there are definite "sex strains." Some 

 strains produce 40 to 50 per cent of males, others produce a very 

 low percentage, 2 to 5 per cent, while others produce no males at 

 all, although reared through as many as seventy-two generations. 



The greater part of the work of the present paper was planned 

 and begun in the spring of 1906, under the direction of Prof. T. 

 H. Morgan, before the results of Punnett were published. Not 

 knowing how to obtain proper food cultures the rotifers all died 

 in July and the continuation of the experiments was deferred until 

 October, 1906. 



