•470 CHARLES R. STOCKAED AND E. M. VICAR] 



The histologic patterns of the pituitary glands in the back- 

 crosses between F, hybrids and the two parental stocks. In 

 certain defective features, the histologic structure of the Fx 

 hybrid pituitary has been shown to approach more nearly 

 the pattern of the Boston terrier than the dachshund. This 

 fact would suggest that some of the arrested and modified 

 tissues in these pituitaries may result from dominant muta- 

 tions within the genes of the Boston terrier stock. Such a 

 suggestion is largely supported by the correlations between 

 head types and rather definite histologic patterns of the 

 pituitaries among the F 2 hybrids, as was discussed in the 

 previous section. In spite of the fact that a large amount 

 of valuable material from this generation was available — 

 sixty-five specimens were observed and measured during life 

 and at autopsy, and the glands from thirty-three of these 

 were studied histologically — the number of F 2 individuals 

 has not been sufficiently large for an accurate genetic analysis 

 of such intricate character complexes. However, we were 

 fortunately able to supplement the study of these F 2 animals 

 with evidence obtained from backcrossing the F x hybrids on 

 the two parent stocks. In the analysis of a complex of modified 

 characters resulting from polygenic mutations, as contrasted 

 with the normal state, the chance of recovering more nearly 

 complete combinations of parental genes is much greater 

 in the backcross hybrids, of course, than it is in the F 2 

 generation. 



Twenty-nine dogs have been produced from the backcross 

 between the F a hybrid and the Boston terrier parent, and 

 nine from the F, backcrossed with the dachshund. The pi- 

 tuitary glands from fourteen of the former and six of the 

 latter have been studied histologically. The individuals among 

 the backcross hybrids on the Boston terrier stock have equal 

 chances of possessing either the Boston terrier pituitary 

 pattern or that of the F, hybrid, and only these two patterns 

 would result if the character depended upon a single gene- 



