150 CHARLES R. STOCKARD AND A. L. JOHNSON 



tutional defects in dentition in some breeds and deficiencies 

 in tooth structure, such as hyperplasia of the enamel and 

 pitting of the tooth surfaces. These dental defects are 

 exactly similar in some cases to those which have been 

 reported as resulting from vitamin deficiencies in the dog 

 and other animals, yet in the present cases these malforma- 

 tions of the teeth are constitutional reactions arising in cer- 

 tain typed animals whose litter mates develop perfectly nor- 

 mal teeth under identical vitamin supply and other living 

 conditions. 



The modified leg condition was found to be a simple single 

 factor dominant expression, but the genetics of the short 

 achondroplasic head modification, such as found in the bull- 

 dog and other breeds, is much more complex. Certain factors 

 in the complex influence only definite distinct parts of the 

 head in either a dominant or a recessive fashion, while others 

 affect entirely different features. These modifications of dif- 

 ferent elements among the structures of the head are in- 

 herited more or less independently, and as a consequence 

 frequent disharmonies occur among the parts, often disturb- 

 ing functional efficiency. The so-called processes of develop- 

 mental regulation are ineffective in bringing about structural 

 adjustments in the hybrids resulting from crosses between 

 breeds with widely contrasted head forms. 



The maladjusted conditions found in the skulls of deformed 

 dog breeds, and particularly of their hybrids, are frequently 

 comparable with well known modifications and distortions 

 which occur in human beings. Structural disharmonies in 

 the growth of the jaws, giving various degrees of dental 

 malocclusion, may be cited as a widely prevalent condition 

 common to both dogs and man. The association of a number 

 of different maldevelopments with modifications of the endo- 

 crine glands and the difficulty in differentiating between cor- 

 relation and causal relation in the interpretation of many of 

 the conditions are considered in the following pages. The 

 strongly contrasted proportions of bodily structures and wide 



