GEXETIG TYPE AND THK ENDOCRINES 361 



disproportionately long 1 and prognathous. Figures 10 and 11 

 are rather well proportioned and intermediate in character 

 though both are slightly undershot. In life, the head of the 

 dog in figure 12 resembled the Pekingese even more than the 

 photograph indicates. This is the only F 2 hybrid that is 

 overshot; the lower jaw is Pekingese-like and, as compared 

 witli the upper, is disproportionately reduced. 



STRUCTURAL DISHARMONY AND FUNCTIONAL MALAD- 

 JUSTMENT BETWEEN THE UPPER AND LOWER 

 JAWS IX BREED HYBRIDS 



The Pekingese-Saluki cross, along with others previously 

 surveyed, fully demonstrates the fact that the discrepancies 

 in pattern between the upper and lower jaws in the mam- 

 malian head are inherited and developed as separate and 

 independent characters. No doubt the strange lack of inter- 

 relation between both the genetics and development of the 

 two jaws underlies the widespread disharmony in facial 

 arrangements commonly seen among race and breed hybrids, 

 one of the most frequent of which is faulty dental occlusion. 

 Further, an abnormal reduction in length of one or both 

 jaws is often associated witli chondrodystrophy in the basi- 

 cranium and other regions and leads one to suggest that 

 cartilaginous growth plays a more important part in the 

 early development and form of the maxilla and mandible 

 than embryologists have generally recognized. Finally, the 

 fact that the upper jaw may develop along one pattern, and 

 the associated lower jaw on quite another plan, leads one to 

 question the validity of the theory that their embryonic 

 origin is from different portions of the same branchial arch. 



A case bearing very directly on such embryological prob- 

 lems may be cited. In the cross between the bassethound and 

 Saluki, two breeds having different skull proportions but 

 without abnormal head distortions, some of the F L . hybrids 

 show very significant jaw arrangements. The Saluki, as al- 

 ready mentioned, has a long slender skull with almost no 

 depression at the nasion. This skull is shown in lateral view 



