132 LEONELL C. STRONG 



IV. CONCLUSIONS 



The conclusions to be drawn from the work here reported 

 naturally group themselves under two headings: A. The activity 

 of the tumor cell; B. The reaction of the host. 



A. The activity of the tumor cell 



1, There is, in some cases at least, a uniform reaction, pro- 

 viding the tumor is transplanted into individuals of the same age 

 and sex of a relatively homogeneous series of hosts. In other 

 words, no rhj^thms of tumor growth are encountered. 



2. A transplanted tumor grows progressively (within limits) 

 at a fairly uniform rate of development if placed in definitely 

 proved homogeneous mice (dBr stock and Fi hybrids). Sudden 

 fluctuations in growth activity may sporadically occur, due 

 possibly to a process analogous to mutation. 



B. The reaction of the host 



1. Race is the primary factor that determines whether or not a 

 given individual shall or shall not grow the tumor mass progres- 

 sively. Susceptibility and non-susceptibiUty are manifesta- 

 tions of the genetic constitution of the individual. 



2. Several secondary physiological factors, among which age 

 is the most important, function in determining the outcome of 

 a given reaction. These may be called contributory or accessory 

 factors. 



3. The age factor is an expression of the degree of the process 

 of the assumption of tissue specificity controlled to some extent 

 by the activity of the gonads. 



4. The age-susceptibility curve towards transplantable tumors 

 for normal individuals of a non-susceptible race bears a remarkable 

 similarity to the curve of activity of the gonads. 



5. The sex factor (encountered especially with young mice in 

 development) depends upon at least two primary causes, 1) 

 the age factor and, 2) the difference in metabolic activit}^ between 

 the sexes, at the diiferent age periods of life. 



