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, 

 V 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 for eight 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- 

 j 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 



