GENETIC TYPE AND THE ENDOCRINES 447 



as dachshund head and upper jaw in association with the 

 Boston terrier short mandible and dachshund legs are mosaic 

 arrangements apparently due to genetic determination of 

 localized growth reactions which break across endocrinic 

 influences. 



Very important questions arise as to the nature of the 

 apparent correlation between the histologic structure of the 

 thyroid and the physical type of the animal. Does the modified 

 structure of the thyroid so alter the internal secretions as 

 to bring about a distorted growth pattern! Or are both the 

 disturbed histology of the thyroid and the modified form of 

 the bulldog head due to an underlying common cause of which 

 these are merely correlated symptoms? Or finally, if there 

 is a common cause of endocrinic nature for both thyroid 

 modification and skull distortion, might we logically suspect 

 the pituitary gland, since there is much evidence to indicate 

 its close interrelation with the thyroid and many investigators 

 would suspect its influence on the growth distortions involved? 

 Because of these questions, a survey of the microscopic struc- 

 ture and possible modifications of the pituitary glands in 

 the Boston terrier-dachshund cross is desirable at this stage 

 of our discussion, before proceeding with the consideration 

 of other breed crosses. 



THE NATURE OF THE PITUITARY GLANDS IN THE BOSTON TERRIER- 

 DACHSHUND CROSS AND THEIR POSSIBLE BEARING ON 



MODIFICATIONS OF THE THYROID AND GROSS 

 STRUCTURAL DEFORMITY 



The canine pituitary is a very unstable organ in both the 

 details of its gross composition and its finer histologic ar- 

 rangements, and this state of structural variation in the 

 pituitary is probably connected in some way with the in- 

 stabilities of physical form, type and size which, in the species 

 Canis familiaris, give rise to the extremely diverse and 

 numerous breeds. Among the mammals, as we have mentioned 

 in an earlier section, only the human species approaches the 



