196 KATHARINE FOOT AND E. C. STROBELL. 



inhibiting factors in either the X chromosomes or the ordinary 

 chromosomes and we are thus forced to admit that inhibiting 

 factors whatever they are must be located outside the chromo- 

 somes in the region of pure hypothesis. The facts force us to 

 consign to these hypothetical inhibitors, not only the responsi- 

 bility of suppressing the spot factors in all the females, but also 

 of determining just how many spot factors shall find expression 

 in the males of the FI and F 2 generations, and thus they prac- 

 tically relieve the chromosomes of the burden of unit distribution. 

 Chromosomes. The small idiochromosome is an exclusively 

 male character a feature it has in common with the genital spot 

 and if it could be put to the same test of experimental breeding, 

 which we have attempted for the genital spot the results ought 

 to show whether this idiochromosome has indeed the individuality 

 which modern chromosome theories demand or whether like the 

 genital spot its relative size is merely a structural feature which 

 can be transmitted through the female. We confidently ex- 

 pected to be able to apply this test to the small idiochromosome 

 of variolarius, for it had been demonstrated that this chromosome 

 in variolarius was relatively much smaller than in servus. 



The chromosomes of Euschistus servus were studied by Wilson 

 in 1906 and he concluded that the XY chromosomes of this form 

 differed from those of E. variolarius in their relative size. He 

 says: "The above described species of Euschistus, while agreeing 

 precisely in the general relations, present individual differences 

 so marked as to show that even the species of a single genus may 

 be distinguishable by the chromosome groups. In this case the 

 most interesting feature is the series shown in the inequality of 

 the idiochromosomes, which become progressively greater in the 

 series (i) E. servus, (2) tristigmus, fissilis, (3) ictericus, (4) vari- 

 olarius, the inequality in the last case being fully as great as 

 Lygaus" (pp. 17). He demonstrates a marked difference in the 

 relative size of the idiochromosomes of E. variolarius and servus 

 in a and c of his Fig. 4. 1 



!\Ve have a large number of photographs of entire groups of chromosomes of 

 E. variolarius and E. servus and we have also preparations from which we can 

 photograph the chromosome groups of the Fi and F2 generations of this cross 

 from both the testes and embryonic stages, but these results are reserved for a 

 later publication, in which we shall give a complete account of our experiments in 

 crossing E. variolarius with both E. servus and E. ictericus. 



