Fasten , Part hen ogenetic Data. 
65 
cause them to develop sexual daughter females, whereas Shull 
believes that the chemicals may act indirectly, by interfering with 
certain internal factors of the organism, that account for the 
appearance of the male producing forms. 
In some of his more recent investigations, Shull (Ti, ’12) 
definitely asserts that the production of parthenogenetic females, 
as well as sexual forms, depends on internal factors of the organ¬ 
ism, and that these internal factors may be indirectly affected by 
conditions of temperature, food, and chemicals, so as to give rise 
to one or the other cycle. Shull thus puts himself in line with 
those who adhere to Woltereck’s point of view. 
All these experiments tend to show pretty conclusively that 
the method of reproduction depends, to a large extent, on condi¬ 
tions surrounding the organism. 
B. Crustaceans .—Among the lower groups of Crustaceans we 
find a number of interesting cases of parthenogenesis. Recently, 
investigators have concerned themselves mainly with the Daphnice 
in which an alternation between parthenogenetic and sexual forms 
is clearly discernible, producing in the first case males as well as 
females, and in the second case females only. 
The winter eggs produce females in the spring, and these 
deposit eggs that develop without fertilization. This mode of 
reproduction continues throughout the summer months, only 
ceasing in the fall, when unfavorable conditions prevail. Now 
eggs are laid from which males and females are produced. These 
copulate to form the winter eggs, from which the parthenogen¬ 
etic females arise the following spring. 
It is rather questionable whether maleness or femaleness is 
brought about through the influence of external conditions. There 
is very strong evidence against any such contention. However, 
from recent investigation we can assert with definiteness that the 
two different modes of development are brought about by changes 
in the environment. 
Kurz, from his work on Daphnias, concludes that the sexual 
phase sets in when the waters become scarce or when they become 
stale. 
Issakowitsch, on the other hand, believes that the sexual 
cycle is brought about through a reduction of the food supply, 
