GENETIC TYPE AND THE ENDOCRINES 255 



homozygosity of head factors in the genotype of the Boston 

 terrier parent. We therefore made a particular effort to 

 employ only carefully bred Boston terriers with fully pro- 

 nounced skull types. The snout indices of the F x skulls ap- 

 proach more nearly those of the dachshund than the Boston 

 terrier. The determination of the short muzzle for the Boston 

 terrier is largely recessive in its expression. 



The eleven F 2 skulls range for snout index from 68 up to 

 97, thus also tending to lie nearer the dachshund than the 

 Boston terrier values and again emphasizing the complex 

 and rather recessive nature of the determining influences 

 which bring about the short muzzle in the Boston terrier. 

 The highest snout index among the eleven F 2 skulls is 39 

 units below the lowest index for the four Boston terrier skulls 

 and only 31 units above the highest dachshund index. In con- 

 trast to this wide divergence, the lowest F 2 index approaches 

 to within 2 units of the range of the dachshund index, while 

 it is 68 units below the lowest Boston terrior index. 



The backcross offspring from the F x hybrid bred with the 

 Boston terrier stock show a considerable range in snout 

 index, from 86 to 133. The highest snout index in these nine 

 skulls approaches close to the pure Boston terrier index, 

 while the lowest is far higher than those of the dachshund. 

 This backcross on the Boston terrier gives expression to 

 much of the recessive quality for the short muzzle. Only one 

 backcross on the dachshund skull is represented in the chart, 

 and, as would be expected, its index is low and approaches 

 the long muzzled type. 



The chart for the palatal indices in these skulls closely 

 follows, though in less pronounced manner, the spread of 

 the snout indices. The three Fj skulls have a closely uniform 

 palatal index value, and the indices for the eleven F 2 hybrids 

 fall almost midway between those for the two parents. The 

 nine skulls from the hybrids produced by a backcross on 

 the Boston terrier show indices only slightly higher than 

 the F 2 levels although they are more uniform in their ap- 



