No. 523] REPRODUCTION OF DAPHNIA 



409 



movements of the daphnids so that they did not keep 

 the algag stirred up in the water sufficiently to get at 

 them. The parthenogenetic egg arises from four cells, 

 but a large number of cells enter into the composi- 

 tion of the fertilizable egg. If the latter egg is not 

 fertilized it is absorbed and, as Issakowitsch noted, fur- 

 nishes food for the development of parthenogenetic 

 eggs. 



Woltereck 12 found that starving hastened the appear- 

 ance of sexual re] induction, hut a concentration of food 

 above the optimum produced results similar to starving. 

 He found, as Weismanh maintained, a cyclical tendency 

 toward the alternation of sexual and parthenogeneii? 

 generations which, contrary to "Weismann, was tempo- 

 rarily influenced by nutrition, and the effects of constant 

 nutrition over a long period was inherited to some ex- 

 tent. 



Langerhans 13 found that the accumulation of excre- 

 tions caused the decrease in numbers of parthenogenetic 

 females in the autumn and thinks that the appearance 

 of sexual forms is due to the same cause. 



The life cycle of a daphnid is, therefore, an hereditary 

 tendency, but can be influenced by nutrition and probably 

 by temperature and the accumulation of excretions. 

 Xutrition is the most important factor, and former ex- 

 periments on the effect of temperature and the drying 

 up of the water were complicated by secondary effects 

 on concentration of food and excretory products. 



Discussion of Results.— Two views might be held as 

 to the origin of the differences between the germ and 

 body cells : the differences might be the result of differ- 

 ence in position in the embryo, or of unequal mitoses. 

 In the parasitic copepods I found the primary germ cell 

 to arise by an unequal mitosis at the fifth cleavage of 

 the egg. The germ cell when first formed is one thirty- 

 second of the total number of cells, but owing to the more 



