HEREDITY AND SEX 



bound to conclude, something occurs, perhaps a fixing of the 

 metabolism-rhythm, perhaps some alteration of the ratio 

 between nucleo plasm and cytoplasm, perhaps the introduction 

 of a specific qualitative sex-determinant in fertilisation, which 

 decides whether the organism will become a male or a female 

 and whether masculine or feminine hereditary characters will 

 find expression. 



Our conclusion in regard to the second theory must be 

 That there is little warrant for attaching much importance to 

 the relative condition of the germ-cells at the time of amphimixis. 

 The experiments of such a careful worker as Richard Hertwig 

 incline one to keep the question open, though O. Schultze's 

 results seem to close it in one case at least. He experi- 

 mented with enormous numbers of mice, which are very good 

 subjects, being ready to breed when seven weeks old, and 

 littering, it may be, every three weeks, if not allowed to 

 suckle. He found that the proportions of the sexes were 

 unaffected by the age of the parents, by apparent vigour, by 

 consanguineous unions, by frequency of births, or by any kind 

 of nutritive change. 



7. Third Theory : That the sex is fixed at a very early stage 

 by the constitution oj the germ-cells as such, there being 

 jemale-producing and male-producing germ-cells, constitu- 

 tionally pre-determined Jrom the beginning. 



On this view there are two kinds of germ-cells, consti- 

 tutionally pre-determined to be female-producers or male- 

 producers. This implies that the sex is determined before 

 fertilisation, thus excluding the second theory. It also implies 

 that the influence of the environment is negligible after the 

 germ-cells have been established, and a fortiori after develop- 

 ment has begun, thus excluding the first theory. 



Kind? of Ova. It may be tha there are two kinds 



