316 T. H. Morgan 
common usage sex-determining means to call forth a male or 
a female. 
Issakowitsch found that by keepingthe daphnian (Simocephalus 
vetulus) at a temperature of 24° C. he obtained during six genera- 
tions a succession of parthenogenetic forms, about 500 individuals 
in all. At the end of three and one-half months the culture died 
out. In another culture carried through six generations composed 
of 85 broods of about 500 animals in all, the great percentage of 
individuals were parthenogenetic, but here and there males and 
winter eggs appeared. ‘The longer the culture was kept at 24° C. 
the stronger was the tendency to produce only parthenogenetic 
females. An examination of Issakowitsch’s table brings out a 
point not mentioned by him, namely, that along certain lines of 
descent there appears to be a stronger tendency to produce males 
than along other lines. 
At room temperature the results are strikingly different, for even 
in the first generation males and winter eggs appear so frequently 
that the cultures disappeared in one or two generations. 
In the cold the cultures were shorter owing to a stronger tend- 
ency to produce males and winter eggs. 
The results seem to show that sex-production is determined in 
these animals by temperature. Food was equally supplied in 
all cultures yet Issakowitsch thinks that in the final analysis food 
conditions bring about the result in connection with temperature 
which only actsindirectly._ Inordertoexamine this question he kept 
two sets of animals under the same temperature conditions viz: 
24° C. One was starved, the other fed. The former produced 
males and sexual females at once. Issakowitsch concludes that 
food rather than temperature per se is the sex-determining (sex- 
producing) factor. He believes that the following facts strengthen 
his view. If the winter egg is not fertilized its contents are 
absorbed, the empty shell being thrown off. Issakowitsch thinks 
that this winter egg supplies the ovary with nourishment. In con- 
sequence the next brood is parthenogenetic. After this the condi- 
tions between temperature and food are again established, and a 
sexual brood results at a low temperature. 
A correlation is found to exist between the kind of egg pro- 
