xi] IDIO-CHROMOSOMES 157 



the male had one heterochromosome and the female two 

 were few, and as far as was known the chromosome-groups 

 of the two sexes in the great majority of animals were alike. 

 It thus seemed difficult to suppose that in a few species sex 

 was dependent on the presence or absence of a particular 

 chromosome while in many others no such difference 

 existed. This difficulty has been to a great extent removed 

 by a discovery made almost simultaneously by Miss N. M. 

 STEVENS and E. B. WILSON (1905-6), and since extended 

 by other investigators to a large number of different species. 

 These American cytologists observed that in certain Beetles 

 and Hemiptera, although both sexes possessed the same 

 number of chromosomes, the male had an unequal pair 

 which behaved in the spireme stages like the heterotropic 

 chromosome, but came together as a bivalent and separated 

 from one another in one of the spermatocyte divisions. In 

 this division it was easy to see that the two members of the 

 pair were of different sizes, so that two kinds of spermatozoa 

 were formed, one having a large chromosome and the other 

 a small one. In the female, however, as in the species in 

 which the male possesses only a single heterp-chromosome, 

 there is an equal pair, both members being of the same size 

 as" the larger member in the male. Chromosomes of this 

 kind WILSON called idio-chromosomes, a term which is often 

 used in a rather more general sense to include the hetero- 

 chromosome as a special case. Frequently these chromo- 

 somes are spoken of as X- and ^-chromosomes; it is said 

 that the male has an X-T pair, while the corresponding 

 female has two ^-chromosomes (PL XVII, 6-9). 



By investigating a number of species WILSON found that 

 he could obtain a series ranging from forms in which no 

 l^-chromosome was present, through those in which it was 

 very small, and others in which it was nearly as large as the 



