158 SEX-DETERMINATION [CH. 



X-chromosome, to the final condition in which the two 

 idio-chromosomes were of the same size. Or, probably more 

 correctly, if we begin with species in which both sexes have 

 two apparently similar idio-chromosomes, we find all stages 

 of the degeneration of one of them in the male until we 

 reach the stage at which the smaller or l~-chromosome has 

 disappeared altogether. We may therefore suppose that in 

 these species the female has always two X-chromosomes 

 while the male has an X-T pair. In many forms X and T 

 are not visibly different and in these the chromosome-group 

 of the male is indistinguishable from that of the female; 

 in others the T is reduced in size, and all stages of its 

 reduction can be found till the stage is reached at which 

 the male has one idio-chromosome (hetero-chromosome) and 

 the female two. In all these cases, therefore, the male may 

 be regarded as producing in equal numbers two kinds of 

 spermatozoa, X- and jT-bearing, while all the eggs bear 

 X, with the result that two kinds of zygotes are formed, 

 XT and XX, of which the former kind will become males 

 and the latter females. On this hypothesis, therefore, those 

 species in whic,h the male has only a single heterotropic 

 chromosome constitute only a special case of a phenomenon 

 which is of much more general occurrence the determina- 

 tion of sex by spermatozoa which either do or do not contain 

 a female-determining X-chromosome. 



This condition in which the male contains either only a 

 single "sex-chromosome," or an unequal pair of idio- 

 chromosomes, while the female has two equal chromosomes 

 corresponding with the larger or "X" element in the male, 

 has now been found in a large number of animals of various 

 groups. In addition to a number of species of Hemiptera 

 (e.g. Protenor, Anas a (Coreidae), Pyrrhocoris, Pentatoma, 

 Lygaeus, Euschistus, etc.) and Orthoptera (e.g. Blatta, 



