Life Cycle of Hydatina Senta 315 



2. In three experiments a rather low rate of egg production 

 (from which partial starvation was inferred) was followed by 

 the appearance of males. 



3. In 12 experiments, where abundant food was supplied and 

 the experiment continued for several days, only females were 

 produced. 



4. In seven experiments, where hunger was evidenced by the 

 small quantity of free food present or the state of fullness of the 

 gut, males appeared later; but in seven other experiments hunger 

 was not followed by the production of males, and in five others 

 males appear without preceding hunger. 



It appears that Nussbaum draws his chief support from the 

 cases of inferred starvation in (i) and (2) above, since those in (3) 

 and (4) are contradictory. As the rate of egg-production varies 

 considerably even with abundance of food, the conclusion that 

 starvation increases the proportion of male-producers rests largely 

 upon the cases where the food of a single aquarium was divided 

 among many individuals. 



The conclusions of Maupas and Nussbaum were tested by 

 Punnett ('06) in several experiments carried out with great care. 

 He isolated each young female and followed its history individ- 

 ually, which neither of the preceding investigators seems to have 

 done. He was unable to secure in three generations an increase 

 in the proportion of male-producers by starving the young 

 females for some hours after hatching. Variations of temperature 

 from 8° to 23° C. yielded no results, though the animalswere kept 

 four to eight days near each extreme. Punnett thought he found 

 evidence, however, of strains, each yielding a rather definite pro- 

 portion of male-producers. He recognized three types of parthe- 

 nogenetic female; one yielding many male-producers (ca. 40 per 

 cent), one few male producers (ca. 2 per cent), and one no male- 

 producers. His general conclusion was that external conditions 

 had no influence on the sex of the offspring, but that this was de- 

 termined by an internal factor, the zygotic constitution. The 

 character of the male and female elements uniting in the winter 

 egg would, according to his interpretation, determine the ratio 

 of male-producers in the parthenogenetic generations that fol- 



