128 LEONELL C. STRONG 



probable error). The third group mentioned above is also 

 slightly below the control group in percentage reactions, but not 

 nearly so resistant as are those from the intermediate group 

 (5 to 10 days). The result is probably due to the operation itself, 

 and not to the removal of the gonads as such. The operation 

 itself may have several effects, one or more of which must ex- 

 plain this increased resistance of animals inoculated five to ten 

 days after operation. 1) Ether is very harmful to the health of 

 the animals. This, however, cannot wholly explain the observed 

 result, since the effect of ether must wear off gradually begin- 

 ning with the first day after operation. 2) The shock and dis- 

 turbing elements of the operation may be so great that the nor- 

 mal physiological activities of the animal are greatly disrupted. 

 Recovery from such an effort would of necessity be slow and 

 gradual. 3) The third probable effect of the operation that may 

 have an influence on the result obtained is the phenomenon of 

 leucocytosis. After an operation, an individual commonly 

 reacts by producing a large number of leucocytes to serve as 

 protective agents. This reaction reaches its height in about 

 seven to ten days. So that in the arbitrarily chosen time group 

 2 (5 to 10 days), we have included those individuals that are at 

 the height of the 'leucocytic' reaction. The leucocytes probably 

 function in protection against all foreign bodies. Is it not rea- 

 sonable to suppose, therefore, that an increased leucocytosis (as 

 a result of the operation) is the chief causative agent in the 

 temporarily acquired resistance to the tumor cell? The lymph- 

 ocytic theory as an all important explanation for tumor im- 

 munity is fast losing ground. The recent conclusion of Sitten- 

 field,". . . . that neither increase nor reduction of the 

 lymphoid elements in the blood had any influence upon either 

 resistance or susceptibility to tumor growth," is being strength- 

 ened by the work of several investigators, especially those at 

 the Crocker Laboratory. 



There is a gradual increase of percentage reactions obtained 

 by keeping individuals from sixteen days to 120 days after opera- 

 tion before inoculation. If animals are kept longer than 120 

 days before inoculation, there is a gradual decline in the number 



