STUDIES ON CHROMOSOMES 91 



Wilson) it consists of two unequal components, always separate, 

 in the diploid groups but closely in contact (not fused) in both 

 spermatocyte-divisions. The recent work of Guyer ('10) indi- 

 cates a similar condition in the X-element of man. In Agalena 

 (Wallace) there are two equal components, always separate. 

 Finally, in Ascaris lumbricoides (Edwards, '10) there are five com- 

 ponents, separate, and scattered in the diploid groups but closely 

 associated in the spermatocyte-divisions. 



In all these cases the significant fact is that not only the number 

 but also the size-relations of these components are constant; and 

 in many of these forms this fact may be seen in such multitudes 

 of cells, and with such schematic clearness, as to leave no manner 

 of doubt. It seems impossible to understand this series of phe- 

 nomena unless we assume that the single X-chromosome is essen- 

 tially a compound body i.e., one that consists of different con- 

 stituents that tend to segregate out into separate chromosomes. 

 We are led to suspect, further, that the composition of the X- 

 element, even when it is a single chromosome, may differ widely 

 in different species because of its great variations of size as between 

 different species. For instance, in the family of Coreidae it is 

 in some cases very large (Protenor), in others of middle size (Che- 

 linidea, Narnia, Anasa), in others one of the smallest of the chromo- 

 somes (Alydus). Similar examples might be given from other 

 groups. 



In the case of Thyanta, therefore, it seems a fair assumption 

 that the double X-element of the B form likewise represents at 

 least a partial segregation of the X-chromatin from other con- 

 stituents; and the latter may plausibly be regarded as represent- 

 ing the 'Y-chromatin' of the original X-member of the pair. 

 In other words, we may think of the triad element as a YY-pair, 

 one member of which is accompanied by a separate X-chromosome. 

 In accordance with this its formula should be X.Y.Y, while that 

 of the A form is XY.Y; and this may also be extended to other 

 forms of similar type. If this be admissible, the male formula, as 

 regards essential chromatin-content, becomes in general XY.Y 

 and the female XY.XY, both sexes being homozygous for the 

 Y-constituents, while in respect to X the male is heterozygous, 



