88 EDMUND B. WILSON 



chromatin and the Y may themselves be composite, thus giving 

 the possibility of many secondary modifications. The point of 

 view thus afforded opens many possibilities for an understanding 

 of sex-limited heredity, as indicated beyond. 



(6) Modifications of the X-element. This view of the XY-pair 

 is based upon two series of facts, of which the first includes the 

 various modifications of the X-member of the pair seen in dif- 

 ferent species. It is, perhaps, most directly suggested by a study 

 of the pentatomid species Thyanta custator. In this common and 

 widely distributed species I have found two races, which thus far 

 can not be distinguished by competent systematists, 7 but which 

 differ in a remarkable way in respect to both the total number 

 of chromosomes and the XY-pair. In one of these races (which 

 I will call the 'A form'), widely distributed throughout the south 

 and west, the total number in both sexes is 16, and the XY-pair 

 of the male is a typical unequal pair of idiochromosomes, exactly 

 like that seen in many other pentatomids (e.g., Euschistus, 

 Coenus or Banasa) . These are shown in fig. 5 a, b, their mode 

 of distribution being the usual one. The second race (the 'B 

 form') is thus far known from only a single locality in northern 

 New Jersey. It differs so remarkably from the A form that I 

 could not believe the observations to be trustworthy until repeated 

 study of material, collected in four successive years, established the 

 perfect constancy of the cytological conditions and the apparent 

 external identity of the two forms. In this race the XY-pair is 

 represented by three small chromosomes of equal size, which are 

 always separate in the diploid groups and in the first spermato- 

 cyte-di vision (fig. 5i), but in the second division are united to 

 form a linear triad series (5 c, d) . This group so divides that one 

 component passes to one pole and two to the other (5 e, Ji) , the 



7 I am indebted to Mr. E. P. Van Duzee for a careful study of my whole series 

 of specimens of both races. He could find no constant differential between them. 

 Additional studies of this material are now being made by Mr. H. G. Barber. 



Addendum. Since this paper was sent to press Mr. Barber, after prolonged 

 study, has reported his conclusion that the 'A form' is Thyanta custator of 

 Fabricius, while the 'B form' is probably Thyanta calceata of Say, which has 

 long been regarded as a synonym of former species. 



