WHAT DETERMINES SEX? 103 



which to base these and similar views has not 

 been supplied. We have, however, two very 

 careful sets of experiments. O. Schultze 

 experimented with enormous numbers of 

 mice, and found that the proportions of the 

 sexes were not affected by differences in the 

 ages of the parents, by differences in apparent 

 vigour, by close inbreeding, or by frequency 

 of successive births. Nor did he find that 

 any kind of nutritive change made any differ- 

 ence. As the outcome of prolonged experi- 

 ments with frogs, Hertwig and Kuschakewitsch 

 found that either over-ripeness or under- 

 ripeness of the eggs (due to artificially delaying 

 or hastening fertilisation) led to a large excess 

 of males. Kuschakewitsch, working with ova 

 of a high degree of over-ripeness, actually got 

 cultures of males only, and with only 4*6 per 

 cent, of deaths. 



It will be seen that these two sets of ex- 

 periments point in opposite directions. For 

 frogs, it appears that the state of the ovum 

 at fertilisation is very important ; for mice, 

 it appears that much may be changed without 

 affecting the proportion of the sexes. It may 

 be that the particular change of condition 

 that does make a difference has been hit 

 upon in the case of frogs, and missed, as yet, 

 in the case of mice. It may be that sex is 

 determined in different ways and at different 

 times at the various levels of the scale of 

 being. 



ARE THERE PREDETERMINED MALE- 

 PRODUCING AND FEMALE-PRODUCING GERM- 

 CELLS ? It is possible that many ova are so 

 constitutionally predetermined that they will 



