490 CHARLES R. STOCKARD AND E. M. VICARI 



of thymic tissue differs greatly among the dog breeds as well 

 as among the individuals of some hybrid generations. A 

 number of cases of thymic death have occurred among hybrid 

 puppies, in some instances being clearly due to acute edema 

 of the thymic body. These findings will be reported in other 

 places. There is little if any evidence at present to indicate 

 that the thymus plays a significant role in the growth of 

 structural types among dogs. However, there is the fact that 

 the large St. Bernard typed F 2 bassethound-bulldog, as we 

 shall show in the next chapter, has a large, meaty thymus 

 accompanying a highly acidophilic pituitary, while litter 

 mates of normal and smaller size are without highly acido- 

 philic pituitaries and possess only small amounts of thymic 

 tissue. Such relations between thymic quantity and acidophilic 

 reaction in the pituitary may be involved in the precocious 

 growth and development of thymic fed rats in the experi- 

 ments of Rowntree and co-workers ('35). 



The study of endocrines from the cross between the Boston 

 terrier and the dachshund clearly indicates several general 

 reactions of considerable importance. In the first place, a 

 cross between two stocks, one of which clearly possesses a 

 defective endocrinic system accompanied by distorted physical 

 type, may give rise to a first generation of hybrids with 

 fairly well balanced physical types and vigorous functional 

 reactions. Individuals of this generation may even be, in 

 some respects, physically superior to either parent stock. 

 In the second place, the offspring from these vigorous first 

 generation hybrids are highly heterogeneous in type ; scarcely 

 two of them are closely alike and the great majority are 

 defective in both their morphologic quality and functional 

 reactions. Prenatal mortality among these F 2 hybrids is high; 

 stillbirths are common and many are viable for only a short 

 time after birth. Harelip, cleft palate, internal hydrocephalus 

 and other developmental arrests and defects are not uncom- 

 mon in this generation. Of the members surviving to adult- 

 hood, some tend to approach in type one or the other parental 

 stock, while others present strange mixtures and combinations 



