GENETIC TYPE AND THE ENDOCBINES 439 



A summary examination of plates 81, 82, 83 and 84 makes 

 it quite evident that the large, strongly developed follicles 

 are present in the thyroids of all the long muzzled dog breeds, 

 while the thyroids of dogs with bulldog typed heads and the 

 dwarf breeds with defective muzzles, such as the Brussels 

 griffon and Maltese poodle, all fail to develop large follicles 

 of regular outline and usually show small, abnormally ar- 

 ranged follicles, often with defective walls. The extrafollicu- 

 lar tissue in the thyroids of the long muzzled dogs consists 

 of the connective tissue supporting net with blood vessels 

 and accompanying nerves, as Nonidez has so clearly described, 

 and a sparse amount of light staining parafollicular cells. 

 In almost all bulldog typed and flat muzzled dogs there is 

 apparent arrest and disturbance of follicular development 

 resulting in irregularly arranged extrafollicular epithelial 

 cells, and in some cases these cells secrete extrafollicular 

 colloid or form abortive follicles or, as in the poodle, show 

 plates and cords of epithelial cells reminiscent of the fetal 

 thyroid gland. 



Our complete survey of pure breed glands is far more 

 extensive than has been discussed or illustrated here, but 

 the additional details are in full accord with what has been 

 presented, and their further discussion is not necessary. 



Whether the peculiarities of the thyroid are constantly or 

 necessarily associated with the characteristic structural de- 

 formities of the several breeds cannot be stated with cer- 

 tainty from such a survey of pure breed thyroids even though 

 some correlation may seem probable. For this reason we 

 have attempted to analyze experimentally the relationships 

 between the histologic nature of the thyroid and the physical 

 type of the animal by crossing pure breeds which are con- 

 trasted in both the histologic patterns of their thyroids and 

 gross bodily types. There should be material in the hybrid 

 generations for determining whether the thyroid pattern 

 of one parent breed might be associated with the structural 

 type of the other breed. For example, in the F^ hybrids 



