ix] HEREDITY AND SEX 129 



irrevocably determined by the factors contained in 

 the germ-cells than is the appearance of an inherited 

 Mendelian character. This view is opposed to the 

 older belief that the sex is determined by nutrition 

 and other environmental conditions acting on the 

 embryo or larva. It may be well therefore to 

 summarise the evidence for the belief that sex is 

 essentially inherited, and not determined by conditions. 

 The evidence is of several distinct kinds. In the first 

 place, in a variety of animals (and in Man in the case 

 of ' identical twins'), whenever one egg divides to 

 form two or more individuals, these are always of the 

 same sex. Secondly, several animals belonging to 

 widely diverse groups produce two kinds of eggs, one 

 yielding males, the other females, and in these cases the 

 sex is clearly determined before development begins. 

 Thirdly, in the Bee and other forms, a fertilised egg 

 yields females, an unfertilised egg, males ; in this case 

 fertilisation appears to determine the sex, but condi- 

 tions operating later have no power to change it. In 

 all these widely distributed cases we have direct 

 evidence that sex is determined from the fertilisation 

 of the egg or before it, presumably by ' sex-deter- 

 minants' present in the germ-cells. 



Evidence of a different nature has been afforded 

 by the study of the development of the germ-cells, 

 especially but not exclusively in Insects. In the 

 nucleus of the developing cell in a number of species 



D. 9 



