322 



Diseases of Poiiltri/ 



normally developed birds and that the growth of the tumor 

 (Rous sarcoma) is slower in the underdeveloped birds. 



It is a well known fact in human pathology that physiolog- 

 ical activity stimulates the growth of some tumors, while 

 physiological decline stimulates others. This is undoubtedly 

 true also of fowl tumors. 



Non-malignant tumors located on external parts may be 



Fig. 72. — Cysto-adenoma on the serosa of the intestine. (After Pickens.) 



removed or they may be left alone. They will not ordinarily 

 affect the health of the bird. Even malignant tumors are 

 apparently not contagious. 



The ultimate nature of the causative agents has not been 

 determined. With our present knowledge it is impossible to 

 explain either the spontaneous origin of these agents within 

 an individual or to account for their transfer from one individ- 



