GENETIC TYPE AND THE ENDOCRINES 235 



enamel hypoplasia. This condition is identical with the hypo- 

 plasia of enamel usually attributed to dietary deficiencies, 

 but such deficiencies cannot be considered the contributing 

 cause in this case, since, as stated above, this animal was a 

 litter mate of the one supplying the skull in figure 4 which 

 contains perfect teeth, and both these animals were reared 

 under identical dietary conditions. They developed in the 

 same uterus, suckled the same mother and fed from the same 

 dish. The environment and diet were fully adequate for 

 perfect dentition in one animal, while under the same condi- 

 tions the second individual produced defective, malformed 

 teeth with deficient enamel. The constitution of the animal 

 itself was no doubt at fault rather than that there were 

 deficiencies of environment. A further consideration of dental 

 defects occurring in abnormal constitutions is given in a 

 chapter beyond. 



Figure 3 in plate 39 shows a puppy with spherical and 

 enlarged cranium and pronounced internal hydrocephalus. 

 The facial features of this puppy were Boston terrier-like, 

 and there was pronounced exophthalmos. Figures 4—7 in 

 this plate illustrate other, very different head and facial 

 features occurring in the second hybrid generation. A large 

 percentage of the F 2 hybrids have a protruding lower jaw, 

 the undershot condition, while a few of them show an op- 

 posite arrangement with a prognathous upper jaw extending 

 beyond the anterior end of the mandible and giving a "rat- 

 nose" or overshot condition. It is strange to find such re- 

 versed arrangements in the skulls of animals derived from 

 the same cross breeding. 



The lengths of upper and lower jaws are inherited inde- 

 pendently, and when a member of a breed with long jaws is 

 crossed with a short jawed dog, the F 2 hybrids show various 

 disharmonious combinations and may have either a long 

 upper jaw associated with a very short mandible or the 

 reverse, a short muzzle associated with a long prognathous 

 mandible. Figure 5 in plate 39 and the photographs in plate 

 40 illustrate the prognathous maxilla or the '■rat-nose" de- 



