390 SENESCENCE AND REJUVENESCENCE 



investigation along these lines/ and while different authorities are 

 not as yet in full agreement, evidence which points to a definite 

 conclusion is accumulating. 



It is well known that the daphnid females produce three kinds 

 of eggs, parthenogenic eggs which produce females, parthenogenic 

 eggs which produce males, and zygogenic eggs which produce 

 females. Both the female- and male-producing parthenogenic 

 eggs develop at once and are commonly known as "summer eggs." 

 The zygogenic eggs, on the other hand, are surrounded by a thick 

 shell and hatch only after a quiescent period which often, but not 

 necessarily, coincides with the winter season, hence they are known 

 as "winter eggs." The problem to which attention has been 

 chiefly directed is that of the relative importance of external and 

 internal factors in determining the production of these three kinds 

 of eggs. Weismann believed that a fixed cycle of generations 

 determined by inheritance existed in each species quite independ- 

 ently of external factors; according to this view a certain number 

 of generations of parthenogenic females were produced, then males 

 developed from parthenogenic eggs and zygogenic eggs were pro- 

 duced, which after a quiescent period developed into parthenogenic 

 female-producing females, and these began the cycle anew. 



Later investigators have found that the cycle of generations is 

 far from being hereditarily fixed and that it can be greatly modified 

 by external factors. Under certain conditions, e.g., with high tem- 

 perature and abundant nutrition, parthenogenic reproduction may 

 continue indefinitely. Other conditions, such as low temperature 

 and lack of food, favor the production of males and zygogenic eggs. 

 In general, males and zygogenic eggs are produced under similar 

 conditions. Moreover, Woltereck has found that after producing 

 males the females may again begin to produce females partheno- 

 genically without producing winter eggs, and the same change may 

 occur even after the production of winter eggs. Kuttner showed, 

 however, that the cycle of generations may occur independently 

 of change in external conditions. 



' Some of the more important papers are the following: Issakowitsch, '06, '08; 

 Kuttner, '09; Langhans, '09; Papanicolau, '100, '10b, '11; von Scharfenberg, '10; 

 Strohl, '07, '08; Weismann, '80; Woltereck, '09, '11. 



