MODIFICATION OF REACTION OF THE HOST 183 



partly conditioned by the activity of the epithelial tissues. The latter therefore 

 help to determine whether the stroma shall be cellular or fibrous in charac- 

 ter ; by stimulating the function of the parenchymatous tissues, hormones may 

 thus indirectly also affect the character of the stroma. It may then be stated 

 again that one of the factors which aids in the survival and function of certain 

 differentiated and therefore sensitive endocrine tissues, is not so much a de- 

 ficiency in function of the corresponding host organ, as a stimulation of the 

 transplant by the effective hormone of the host. The deficiency required may 

 be merely a means of accomplishing a stimulation of the transplant by the 

 hormone. 



In conclusion, it follows from the data discussed in this chapter that in 

 certain organs of the adult host, or in embryonic structures, various special 

 conditions exist, which protect at least, to some extent tissues possessing 

 strange individuality or species differentials from the injurious action of the 

 bodyfluids and cells of the host, but that the nature of these mechanisms is, 

 at the present time, only imperfectly understood. 



