GENETIC TYPE AND THE ENDOCRINES 373 



needed accommodation. The general shape and size of the 

 jaws and teeth differ among the dog breeds, as well as among 

 human races, and the hybrid individuals in both species 

 may include misfits having large teeth in association with 

 small jaws and vice versa. The breed hybrids among the 

 dogs constantly emphasize the structural disharmony and 

 functional maladjustments which necessarily arise from type 

 and race mixtures. 



In the cross between the giant St. Bernard dog and the 

 large great Dane there is considerable variability and ir- 

 regularity in the occlusion and relative positions of the upper 

 and lower incisors. For these reasons a litter of seven F 2 

 hybrids from this cross was selected as material for a study 

 of the growth of the jaws and the final establishment of 

 dental occlusion (pi. 78). 



All puppies at birth have fetal-like flat faces with only 

 slight prominence of the muzzle. The upper and lower jaws 

 are of about equal length, and there is little evidence of the 

 pronounced prognathisms which are later to develop in many 

 of the animals ; the growth of the jaws to give the typical 

 muzzle as w r ell as the modified conditions which we have seen 

 is largely or entirely a postnatal process. The discoordinated 

 growths in the two jaws of the modified breeds begin to be 

 noticeable during the first few weeks of age, and at about 8 

 weeks, when the deciduous teeth have appeared, the under- 

 shot and overshot prognathism can be very reliably diagnosed. 

 The type of occlusion present at this time may remain 

 permanently, or may gradually change due to inequality in 

 the further growth of the two jaws. 



The late growth reactions are well illustrated by the various 

 conditions of incisal occlusion in the litter of seven great 

 Dane-St. Bernard hybrids. Photographs were made of the 

 incisor teeth of these puppies at 6 months of age, just after 

 the permanent dentition was completely established. The 

 animals were killed 1 year later, when 18 months old and 

 fully grown, and second photographs of the incisal occlusions 



