224 CHARLES R. STOCKARD AND A. L. JOHNSON 



breeds with strongly pronounced head characters and cross- 

 ing these with standard wolf-like long-skull breeds showing- 

 no symptoms of the short bulldog- head. In the study of so 

 complex a condition as the development of the skull, it has 

 been necessary to resort to several different breed crosses, 

 just as was done for the study of the achondroplasic leg- 

 conditions discussed in previous chapters. 



Crosses Between Dog Breeds With Highly Contrasted 

 Types of Skulls 



The foregoing survey of skull measurements and indices 

 for the different pure breeds of dogs emphatically demon- 

 strates that the skull is a complex of structures, many of 

 which are quite independent in their mode of expression. It 

 is found, for example, that two parts intimately related in 

 function, such as the upper and lower jaws, may develop 

 along very different lines, causing extreme structural dis- 

 harmony and consequent impairment of efficiency. Also such 

 closely related parts as the maxilla and the teeth which it 

 supports may be strangely out of accord. Maldevelopment 

 of the nose and upper face rendering respiration difficult 

 and noisy is a constant characteristic in certain dog breeds. 



These and a number of other less marked distortions re- 

 sult as peculiar growth reactions distinctly deviating from 

 the original canine pattern. Our general problem is to de- 

 termine whether such growth distortions are the results of 

 modifications in the endocrine systems of the different breeds 

 or whether they are due to genetic mutations which give rise 

 to such developments even in a normal endocrine environ- 

 ment, or again, whether there may be a genetic linkage or 

 association between these structural distortions and modi- 

 fications of one or several of the endocrine glands. 



Early surgical removal of endocrine glands and the feeding 

 of gland products or the injection of glandular extracts 

 cannot supply conclusive answers to the above problems. 

 Such experimental procedures have yielded most valuable 

 results tending toward an understanding of the influences of 



