314 Diseases of Poultry 



In the three thousand hens under one year old no maHgnant 

 tumors were found. 



There are many theories regarding the origin of tumors. 

 One which has been quite generally accepted being that 

 unused embryonic cells may remain collected in certain spots 

 and that they may later be sufficiently stimulated to grow 

 independently. In accord with this theory is the fact that 

 the histological structure of a tumor resembles one or another 

 of the general classes of body tissues, and in fact usually 

 copies more or less closely the structure of the organ in which 

 the primary tumor arises. However, when growths arise 

 secondarily in other organs, that is, when the tumor has 

 undergone metastasis, the secondary tumors are similar 

 in structure to the primary tumor. Further Rous and his 

 colleagues ^ have found several distinct chicken tumors 

 which may be produced in healthy fowls by the injection of a 

 cell free filtered extract of the tumor. In the case of each of 

 these tumors the neoplasm produced in the inoculated fowl 

 always resembles the tumor from which the extract was made. 

 That is, the type of tumor is determined by the individuality 

 of the causal agent and not alone by the potentialities of the 

 stimulated cells. That is, the cells are not simply stimulated 

 to grow, but they are stimulated to grow in a specific 

 way. 



According to the general type of body tissue they resemble, 

 tumors may be classified as follows : 



1 Rous, P., "A Transmissible Avian Neoplasm (Sarcoma of the 

 common fowl)." Jour, of Exper. Med., Vol. XII, pp. 696-705. 



Rous, P., Murphy, J. B., and Tytler, H. W., loc. cit. 



Rous, P., and Lange, Linda B., "The Characters of a Third Trans- 

 plantable Chicken Tumor Due to a Filterable Cause. A Sarcoma 

 of IntracanaUeular Pattern." Jour. Exper. Med., Vol. XVIII, pp. 

 651-664, 1913. 



Rous, P., and Murphy, J. B., "On the Causation by Filterable 

 Agents of Three Distinct Chicken Tumors. ' ' Jour. Exper. Med. , Vol. 

 XIX, pp. 52-68, 1914. 



