l6 SEX-DETERMINATION 



I943» 1948, 1950; Matthey, 1949; Oguma, 1946) has shown 

 that not all the Praying Mantis have this type of sex- 

 chromosome mechanism, the males of many genera being 

 XO. It has been suggested (White, 1941; Hughes- Schrader, 

 1950) that the compound type arose from the more primi- 

 tive XO type through a structural rearrangement or series 

 of rearrangements. Possibly a single mutual translocation 

 between a metacentric X and a metacentric autosome con- 

 verted the original XO mechanism into the X^XgY mechan- 

 ism without any intermediate steps. If this is so, then the Y 

 is the homologue of the autosome involved in the trans- 

 location. In the grasshopper Paratylotropidia hrunneri King 

 and Beams (1938) found the sex-chromosome mechanism 

 likewise to be XiX2Y=male, XiXiX2X2= female. 



The Sex-Ratio. The homo- heterogametic mechanism 

 described above should yield a sex-ratio among the newly 

 conceived, a primary sex-ratio, of equality. This numerical 

 relationship of males and females is expressed either as so 

 many males per 100 or per 1,000 females within the group 

 or else as the percentage of males among all the newly 

 conceived. It is impossible to determine the primary sex- 

 ratio among such forms as fishes, birds and mammals by 

 direct observation. It is necessary to examine a sufficient 

 number of foetuses and embryos as near to the point in 

 their development at which the differences that distinguish 

 the sexes can be recognized. When this is done it is found 

 that the sex-ratio among them is not one of equality though 

 not far removed therefrom. This 'foetal' sex-ratio ranges 

 from 44-5 per cent, males in some of these studies to 56-8 in 

 others. {Handbook of Biological Data.) 



These observations do not destroy the validity of the 

 argument concerning the homo- heterogametic mechanism. 

 They are to be explained in different instances by one or 

 other of the following phenomena: 



1. The two forms of gametes elaborated by the hetero- 

 gametic sex are not always produced in equal numbers. 



2. These two forms are not invariably equally efficient 

 fertilization. 



