STUDIES ON ASPLANCHNIA EBBESBORNII. 195 



of the male producers would be rather low and remain constant 

 regardless of external conditions. But if we include the resting 

 egg producers (potentially male producers) in the sum total of 

 the male producers the percentage under favorable conditions 

 often reaches 90 to 95 per cent. 



In this particular rotifer there are two kinds of resting eggs, 

 which require fertilization before development begins. The one 

 (single shelled) develops within the uterus of the parent with 

 the same rapidity as the parthenogenetic forms and hatches out 

 immediately when deposited. The other (double shelled) with 

 the exception of a few early cleavage stages, develops after 

 deposition. 



No attempt was made to regulate the food conditions of the 

 different cultures, except the adding of tap water at different 

 times to counteract evaporation. The amount of food available 

 for the rotifers varied greatly during the active periods of the 

 different life cycles. Some of the cultures were almost entirely 

 free from food of any kind, while other cultures were rich in 

 euglenae and unicellular plants or animals. It was found that 

 the number of male- and female-producing females varied accord- 

 ing to the amount and kind of food present. Scarcity of food 

 favored the production of female-producing females. Culture 

 rich in euglenae and unicellular plants, favored a high production 

 of male-producing females. 



Experiments in mating and sex determination have been 

 carried on extensively by Shull, Whitney and others but will not 

 be considered here, since they do not come directly within the 

 domain of the present investigation. However, some very sug- 

 gestive features on sex determination present themselves in the 

 study of the above rotifer. It is true that certain food conditions 

 favor a high production of male-producing females, but imme- 

 diately after impregnation, as stated above, these same male- 

 producing females begin the production of resting eggs, or both 

 male and resting eggs, which may alternate in the same female. 

 Again it is not an unusual thing to find within the oviduct of the 

 male-producing female embryo before birth, either mature male 

 eggs or male eggs that have begun to develop, and if impregna- 

 tion of these male producers occurs after birth, resting eggs 



