WHAT DETERMINES SEX? 105 



that fertilisation may be necessary if a female 

 is to develop. 



In about thirty different kinds of animals 

 there are two kinds of spermatozoa differing 

 in detail of form, but the significance of this 

 dimorphism is not known. Of greater interest 

 is the fact that in a considerable number of 

 animals, especially insects and arachnids, 

 half of the spermatozoa have in their nucleus 

 the same number of nuclear rods or chromo- 

 somes as the ova have, while the others have 

 one fewer. This extra chromosome, which 

 half have and half have not, is called the X- 

 element or accessory chromosome, and there 

 are facts which go to show that the presence 

 of two X-elements in the fertilised ovum 

 implies development into a female organism, 

 while the presence of only one means a male. 

 It is possible that the amount of chromatin 

 is of some importance, the ova with more 

 chromatin developing into a female. It is 

 possible, as E. B. Wilson suggests quite 

 provisionally, that the X-element contains 

 factors (ferments or hormones?) that are 

 necessary for the development of maleness 

 or femaleness, and that they are so adjusted 

 that in the presence of a single X-element 

 the male character finds expression, while in 

 the presence of two X-elements the female 

 character is liberated. 



THE MENDELIAN VIEW OF SEX. In the 

 volume on EVOLUTION a brief reference has 

 been made to the important discovery known 

 as Mendel's Law which formulates the mode 

 of inheritance of certain kinds of charac- 

 ters called unit characters or Mendelian 



