i9ii] A. Franklin Shnll 175 



chemical substances in the water play a role in determining the 

 cycle, other external agents (temperature and nutrition) have 

 been found inadäquate to explain the observed phenomena. The 

 rotifer most extensively used in the study of the influence of ex- 

 ternal agents on the mode of reproduction is Hydatina senta. 

 Without taking into account facts not directly involved in the 

 present problems, the life cycle of this rotifer may be stated as 

 follows : From the resting (fertilized) egg there hatches invariably 

 a female. This female may lay 40 to 50 eggs which are incapable 

 of fertilization and always hatch into females. In this family, 

 however, the females may be of two kinds. One kind, like their 

 mother, produces only eggs that can not be fertilized and hatch into 

 females; the other produces only eggs that can be fertilized, but if 

 not fertilized, nevertheless develop and produce males. The fe- 

 males may therefore be spoken of as female-producers or male- 

 producers. The former are parthenogenetic females ; the latter 

 may be called sexual females, for, while their eggs may develop 

 parthenogenetically, they may on the other hand be fertilized. Both 

 male-producers and female-producers may occur in the same fam- 

 ily, but the Proportion of each is subject to wide fluctuations. A 

 family ma}'^ consist entirely of female-producers, or (less com- 

 monly) only of male-producers, while every gradation between 

 these extremes may be found. Maupas ('91) referred these great 

 fluctuations to temperature differences, Nussbaum ('97) to nutri- 

 tion; but Punnett ('06) and Whitney ('07) were unable to confirm 

 either of these conclusions. It seemed highly desirable, therefore, 

 to examine the possible influence of the chemical composition of 

 the medium upon the life cycle. The following pages give the 

 results of experiments which show that certain substances may 

 exert a more or less marked influence upon the proportion of male- 

 producers (sexual females). 



II. DESCRIPTION OF THE EXPERIMENTS 



I. Influence of substances introduced with the food on the 

 percentage of male-producers. In the work of previous investi- 

 gators, the results of which led to contradictory conclusions regard- 

 ing temperature and nutrition, no account was taken of possible 



