SEX-CHROMOSOMES 15 



greatly from species to species; the Y may be exceedingly 

 small or it can be equal in size to the X. Among the bugs 

 there are species which can be graded according to the 

 relative size of the Y, from those in which the Y is as large 

 as the X to those in which the Y is absent. 



Hetero- or di-gamety is not a property of the male, how- 

 ever. In birds and lepidoptera it is the female that is hetero- 

 gametic and the male that is homogametic. 



The male of the domestic fowl has an even number of 

 matched chromosomes, including seven pairs of large ones 

 and fifteen pairs of small ones. Sokolow, Tiniakow and 

 Trofimov (1936) concluded that the sex-chromosome was 

 a V-shaped chromosome with arms of equal length which 

 was present in duplicate in the male and in the simplex state 

 in the female. They found it impossible to decide whether 

 or not one or other of the many small chromosomes was the 

 Y in the female. Pheasants, peafowl and turkeys were found 

 to have the same kind of sex-chromosome constitution. In 

 the guinea-fowl and woodcock there were two such chromo- 

 somes in the female, four in the male. 



Among the lepidoptera Seller (1920) found in the moth 

 Talaeporia tuhidosa 60 matched chromosomes in the male 

 and 59 in the female which synapsed into 29 pairs and an 

 unpaired univalent. 



The difference between male and female heterogamety in 

 no way affects the functioning of the mechanism. 



Males Females 



The compound sex-chromosome mechanism has been 

 encountered in Tenodera, Paratenodera, Mantis, Stag- 

 momantis and in Hierodula, the males being X^XaY and the 

 females X1X1X2X2 (Oguma, 1921; King, 1931; Asana, 1934). 

 More recent work (White, 1938, 1941; Hughes- Schrader, 



