NATURAL RESISTANCE 173 



conveys the idea with which we are all familiar, — that 

 individual internal factors over which we have little 

 control determine the physiological activity of an 

 organism. 



Thus, it is well known that captive wild animals 

 do not breed well. Commonly this fact has been at- 

 tributed to constitutional alterations arising from 

 changed feeding habits and inaction enforced by cag- 

 ing which play an important role in their altered 

 breeding activities and the viability of their ofifspring. 

 We have seen previously that these same factors lead 

 to a liberation of cytost and various pathological con- 

 ditions which may be attributed to the action of this 

 substance. Consequently, it became of interest to ascer- 

 tain whether or not immunization to cytost would re- 

 sult in an increased fertility and possibly in a greater 

 viability of the ofifspring. As captive wild animals 

 were not available to the writer, he was forced to con- 

 duct his experiments upon domesticated and labora- 

 tory animals. 



A series of pullets which had not begun to lay eggs 

 were divided into three flocks. The first flock (A) of 

 ten received a course of four Immunizing injections 

 of chicken cytost, spaced one week apart. The sec- 

 ond flock (B) of twenty-five received similar in- 

 jections throughout the period of observation, while 

 the third flock (C) of ten were held as controls. The 

 daily egg production of each flock was noted over a 

 period of seven months, from January through July. 

 During the first three months the number of eggs pro- 

 duced was about the same, although the members of 

 flock A (the immunized birds) yielded a slightly 

 greater number of eggs. At the end of the fourth 



