64 Bulletin Wisconsin Natural History Society. [Vol. 10, Nos. 1 & 2. 
eggs were fertilized, producing females only, whereas the summer 
eggs developed without fertilization, giving rise to males and 
females (each parthenogenetic individual is capable of producing 
eggs developed without fertilization, giving use to males and 
eggs of but one sex, and not both). Among the Crustaceans and 
some of the Insects we meet with a similar state of affairs. 
Recently, interest has chiefly centered on the experiments 
carried on with the Rotifier, Hydatina senta. In this case the 
method of reproduction seems to be influenced by external condi¬ 
tions. 
Maupas, ( ’90) noted that when cold weather set in, the sexual 
cycle appeared, and he came to the conclusion that temperature 
regulated the parthenogenetic and sexual phases of reproduction. 
Nussbaum (’97), stated that these two cycles were controlled 
by the food supply of the surrounding medium. When cold 
weather arrives, most of the food is killed off, and the sexual cycle 
makes its appearance. In summer, food is plentiful and then 
Hydatina continues its reproduction through parthenogenesis. 
Punnet (’06), and Whitney (’07), in an extended series of 
experiments, in which they subjected Rotifers to different condi¬ 
tions of food and temperature, failed to confirm the conclusions of 
either of these investigators. 
Shull ( 09, To), claims that the presence or absence of certain 
chemicals in the environmental medium of the rotifer is the cause 
for the appearance of one or the other modes of reproduction. 
The presence of these chemicals induces parthenogenesis, their 
absence causes the sexual cycle to appear. 
Whitney (To), partially confirmed Shull’s results. From his 
experiments he concluded that chemical substances produced in 
the culture medium of Rotifers so affect the parinenogenetic 
females as to cause them to produce- sexual daughter females. 
When these chemical substances are absent, no sexual females are 
produced, but only parthenogenetic forms. 
Whitney and Shull both believe that external conditions are 
accountable for variations in the sexual and parthenogenetic 
cycles. The former worker, however, is of the belief that there 
are certain transitory chemical substances in the culture medium of 
Rotifers, which directly affect the parthenogenetic females, and 
