J. W. H. Harrison 33 



due attention to the predominantly female nature of the insect, one 

 would have anticipated that it would have been arrow-head shaped. 



On the Causes of the Development of such Intersexes. 



Before we consider the raison d'etre of the present series of inter- 

 sexes the course of events in the oogenesis of the pure species of this 

 subfamily must be examined. 



At a very earl}^ stage indeed in the oocytes destined to result in 

 ova, the homologous chromosomes pair ; amongst those so doing are the 

 sex chromosomes. If the female, as in our group, is heterozygous for 

 sex, then the X chromosome behaves as the homologue of the Y and 

 pairs with it in synapsis. All of the chromosomes remain paired until 

 long after the yolk has been laid down and the egg extruded from the 

 oviduct — nay, even after the spermatozoon has entered it. A division 

 spindle is finally formed and the chromosomes separate, one half set 

 proceeding to one pole and the other half set to the other pole of the. 

 spindle. From this, it must be obvious that only one of these daughter 

 nuclei contains the X chromosome and one the Y when the female is a 

 sex heterozygote. 



Immediately these reach the poles, they form two new spindles in 

 which, instead of having paired chromosomes at the equatorial plate, 

 we have single chromosomes which then split as in the ordinary division 

 of a somatic cell. The outermost pair of little new cells, composed of 

 but little else save chromatin, are extruded as the first polar body. A 

 similar fate awaits the outermost group of chromosomes formed as 

 a result of the division of the original inner series of whole chromo- 

 somes ; it forais the second polar body. It will now be perfectly clear 

 that the innermost nucleus of all, which is the real egg nucleus and that ' 

 which fuses with the spermatozoon, is furnished with an X or a Y 

 chromosome, but not with both. Therefore, on fertilisation by the sperm 

 derived from a male sex homozygote, it can only yield in the normal 

 course of events one fixed sex. 



But whether any given sex chromosome passes in the first maturation 

 division to the outer or inner pole of the spindle is purely a matter of 

 chance ; whence it happens that, since two chromosomes enter into the 

 matter, the chances that any particular egg is endowed with an JT or F 

 are equal. Consequently, the zygotes formed consist of approximately 

 equal numbers of those of male and of female type. 



Turning now to the oogenesis of hybrid females, such as our pilzii, 

 what differences are perceptible ^ 



Journ. of Gen. ix 3 



