344 Experimental Zoology 
At the higher temperature three successive parthenogenetic 
female broods appeared. In the cold the females brought forth, 
in the one case, first parthenogenetic forms, then males twice, 
and then winter eggs twice. In the other case (the right-hand 
table) the offspring of five sisters kept in the cold produced win- 
ter eggs twice and then died. These results show that at high 
temperatures only or largely parthenogenetic females appear, 
while at a lower temperature the sexual method of reproduc- 
tion for the females and the appearance of males occurs. 
Issakowitsch attempted to determine whether temperature 
acts directly or only indirectly by affecting the condition of 
nourishment. Females containing eggs in the brood pouch were 
selected in order to be certain that from the beginning of devel- 
opment the young themselves were subjected to a condition in 
which food was insufficient. These animals were put into pure 
filtered spring water and left there foreight days. This brood after 
hatching produced always sexual animals, males, and females 
bearing winter eggs. The results show that even at the high 
temperature the sexual part of the cycle appears if food is lack- 
ing. Issakowitsch concludes, therefore, that the animals kept 
in the cold produce sexual forms, not because of the cold, but on 
account of the influence of the cold on certain conditions of nour- 
ishment in the ovary, and he points out the following facts that 
seem to support his point of view. In the first place, even in the 
cold if an ephippium is first produced and then thrown off (fer- 
tilization not having occurred) the next brood consists of par- 
thenogenetic females, whatsoever the other external conditions 
may be. Issakowitsch offers the following interpretation of the 
change. If the winter egg in the ephippium is not fertilized, 
it is absorbed, and furnishes the ovary with food materials. 
Owing to this better condition of nourishment the next genera- 
tion is parthenogenetic. After this brood has been set free the 
equilibrium between temperature and food is again established, 
and external conditions will determine what the subsequent 
events will be. 
There are certain peculiarities in regard to the origin and 
