GEXETIC TYPE AND THE EXDOCEINES 495 



this thyroid resembles somewhat more closely that of the 

 bassethound than of the F t hybrid. The thyroid from the 

 St. Bernard typed brother (fig. 6) does not clearly suggest 

 either parental pattern, nor yet that of the F : hybrid. This 

 gland has both large and very small follicles with flat, in- 

 active epithelial walls, and there is no clear resemblance 

 between it and the thyroid gland of the St. Bernard. 



It is well recognized, of course, that the height of the fol- 

 licular epithelium and the apparent thyroid activity as seen 

 in the thyroid section depend upon the condition of the animal 

 when killed. As far as was practical, all animals were killed 

 at a time when they were in normal states of activity. Through 

 close acquaintance with the living animals we do have a 

 definite advantage in knowing the comparative active and 

 inactive tendencies of the dogs concerned, and on this basis 

 can predict with some degree of accuracy the functional 

 symptoms to be found within the thyroid. However, the 

 estimates of thyroid types must in all cases be based largely 

 upon definite differences in histologic pattern, such as those 

 we have pointed out between the bulldog types and the long- 

 muzzled breeds. Yet some importance must necessarily be 

 attached to apparent differences in functional activity of the 

 thyroids at the time of the animal's death. 



Microscopic sections from five other thyroid glands of the 

 F2 hybrids are illustrated in plate 94 to show not only how 

 completely the histologic patterns may revert to the basset- 

 hound and bulldog types but also the occurrence of new and 

 unusual types. The upper left section, from 1055 $ , presents 

 the exact histologic picture of the thyroid gland from a 

 bassethound; the follicles in limited regions are small, but 

 in general are large with walls of low cuboidal epithelium, 

 and the colloid is stale and brittle. As shown in plate 59 

 (fig. 3) (p. 324), the animal from which this specimen was 

 taken is of general bassethound type although the head is 

 somewhat short and heavy, partly bulldog-like. Figures 2 

 and 3 approach the distorted histology characteristic of the 



