396 Experimental Zoology 



same batch. In any case it is not clear, even if large eggs pro- 

 duce fewer males and small eggs more males, whether the result 

 is due simply to the size determining the sex, or whether 

 the female eggs tend to become larger than the male eggs. 

 If analogy has any value in this instance, it seems more 

 probable, from the cases of Phylloxera, Dinophilus, and 

 Hydatina, that sex may be sometimes predetermined in the 

 egg and this determines its size. The question needs, however, 

 a reexamination. 



Ratio of Nucleus to Cytoplasm 



In a recent contribution Richard Hertwig has argued that 

 sex is determined by the relation of nuclear size to cell size. He 

 bases his argument on a supposed analogy with certain condi- 

 tions found in the protozoans, and on some experiments of 

 his own and of his pupils on Dinophilus, Daphnia, and Frogs. 

 The experiments on Daphnia have been already given. Hert- 

 wig thinks that after a succession of parthenogenetic generations 

 the nucleus in the egg-cell slowly increases to judge by 

 analogy with the protozoa and this leads toward the pro- 

 duction of males. This tendency may be held in check by 

 a high temperature, but if the temperature is lowered, males 

 appear. The argument is clearly based on a highly hypothetical 

 comparison, and lacks direct observation in its support. Issa- 

 kowitsch's experiments seem to show that lack of food and not 

 temperature determines the turning point hi the life-cycle of 

 Daphnia. 



Hertwig's experiments with frogs are very inconclusive, and 

 so far as they go lend little or no support in favor of his view. 

 Eggs of Rana esculenta, that had been forced to premature ripen- 

 ing, fertilized by a male already at its full sexual ripeness, gave 

 the following results. Of 40 individuals that reached the frog 

 stage, all were males. There was, however, a high rate of 

 mortality, and the results may only mean that all the females 

 died. In another experiment some eggs were fertilized at once, 

 others after 8 hours. The latter Hertwig thinks must have been 



