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. 
(b) Modtfications 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,? 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. 5a, 6, 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-division (fig. 57), 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 (5e, h), the 
7T 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. 
