GENETIC TYPE AXD THE ENDOCRINES 441 



these two thyroids, the differences which were pointed out 

 on previous pages. In the dachshund, the follicles vary more 

 widely in size, some of them being very large. The epithelium 

 is low cuboidal and less active in appearance, and there is 

 much less extrafollicular tissue in the dachshund than in the 

 Boston terrier where we find an excessive abundance of para- 

 follicular cells and no very large follicles. In other parts of 

 this Boston terrier thyroid there are large confluent col- 

 loidal masses as if two or more follicles had joined. The 

 Boston terrier thyroid approaches the infantile type but is 

 quite active, and the dog itself is nervous and very excitable 

 and shows varying degrees of exophthalmos. The dachs- 

 hund is entirely different from the Boston terrier in disposi- 

 tion and behavior. 



The F! hybrids from this cross, as we have seen in plate 

 29 (p. 127) are of uniform type, and somewhat intermediate 

 when compared with the two parent breeds although they 

 incline toward the dachshund in body form and the Boston 

 terrier in cranial shape. A photomicrograph of a typical 

 section of F x thyroid is seen in figure 3 (pi. 85). The variation 

 from large to very small sized follicles, and the type of extra- 

 follicular tissue, resemble more closely the histology of the 

 dachshund thyroid than that of the Boston terrier. It may 

 be recalled that the relative size of the F x thyroid is also 

 in the direction of the dachshund. The thyroid glands from 

 the five F! individuals studied are fairly uniform. 



Thyroids from more than thirty F 2 hybrids have been 

 examined and studied histologically. These form an almost 

 complete series, with the fully typical dachshund thyroid at 

 one end and an exaggerated or over-expressed Boston terrier 

 gland at the other. Figures 4 and 5 (pi. 85) are striking- 

 illustrations of this fact, figure 4 being histologically extreme 

 for the dachshund and figure 5 an over-expression of the 

 Boston terrier thyroid. 



The thyroid in figure 4 is from 620 $ , which is shown from 

 life in plate 39 (fig. 5) and plate 40 (fig. 1) (pp. 236 and 238). 



