446 CHARLES I!. ST0CKARD AND E. M. VICAEI 



of the parafollicular epithelioid cells so typical of the Boston 

 terrier gland. As we shall find beyond, this modified thyroid 

 from an animal with deformed Boston terrier typed head is 

 accompanied by an extremely distorted and cystic pituitary 

 gland, illustrated by figure 2, plate 89. 



These six sections of F 2 thyroid glands (pis. 85, 86 and 89), 

 would lead one to believe that the histologic patterns of the 

 thyroids in the dachshund and Boston terrier are definitely 

 hereditary characters which, when the breeds are crossed, 

 become redistributed among the F 2 hybrid individuals to 

 form a series of intermediate conditions with the almost fully 

 expressed parental types at opposite ends. The different 

 histologic patterns of the thyroids appear to be closely as- 

 sociated with the structural characteristics of the parent 

 stocks. The development of the Boston terrier bulldog head 

 is definitely associated with the histologic pattern of the 

 Boston terrier thyroid and we have the same association of 

 physical type and gland histology in the dachshund. 



Further evidence of this correlation between physical type 

 and thyroid pattern is shown by the backcross hybrids result- 

 ing from mating the F, hybrid and the pure Boston terrier 

 parent. In appearance these backcross hybrids, as shown in 

 plates 43 and 44 (pp. 245 and 247), are of Boston terrier type, 

 particularly in head form. The two photomicrographs in 

 figures 3 and 4 of plate 86 are sections of the thyroids from 

 524 6 and 525 9 , two backcross Boston terrier hybrids. These 

 photomicrographs show exactly the same microscopic pattern 

 as that of the pure Boston terrier (fig. 1, pis. 82 and 85). 

 Four sections from different thyroids could scarcely show 

 closer similarity. The pituitary glands accompanying these 

 Boston terrier-like thyroids are seen in plate 90 (figs. 1 and 

 2); these are very cystic and abnormal. 



The genera] correspondence between head type and the 

 pattern of thyroid histology does not hold for all similar 

 structural modifications in other parts, such, for example, 

 as the screw-tail in the bulldog. Structural disharmonies such 



