xi] HETERO-CHROMOSOMES 155 



two sexes, so that if we letter them A, B, C, . . . both male 

 and female will contain in each nucleus A, A, B, B, C, C,. . . . 

 But it was discovered by several investigators about the 

 beginning of the present century that in the males of certain 

 insects of the orders Hemiptera and Orthoptera one chromo- 

 some behaved differently from the others during the stages 

 preceding the maturation divisions of the spermatozoa. 

 This chromosome, at first known as the accessory chromo- 

 some, was found to take no part in the spireme and synizesis 

 stages ; while these stages are in progress it remains a com- 

 pact deeply staining body which was sometimes described as 

 a chroma tin nucleolus. When the other chromosomes emerge 

 from synizesis and contract to form the double chromo- 

 somes of the first maturation division, the "accessory" also 

 gives rise to a definite chromosome which takes its place on 

 the spindle with the rest. It may differ from the others in 

 size, being either larger or smaller, but this is by no means 

 invariable. When the bivalent chromosomes divide into 

 their component units and pass to the poles, the accessory 

 usually passes undivided to one pole of the spindle, and 

 hence has been called the heterotropic or hetero-chromosome, 

 the name by which it is now most frequently called. Very 

 often, however, especially by American writers, it is known 

 simply as the X-chromosome. When, as most frequently 

 happens, the heterotropic or ^T-chromosome passes undi- 

 vided to one pole of the first spermatocyte division-figure, 

 it divides equationally in the division of those secondary 

 spermatocytes which contain it. In the Hemiptera the pro- 

 cess is reversed; it divides longitudinally in the first sper- 

 matocyte division and then passes over undivided in the 

 second division into half the spermatids. In either case, 

 therefore, since this chromosome passes to one pole without 

 division in one or other of the spermatocyte mitoses, it 



