Tumors. 321 



Tumors of the third group are formed of muscle-Hke tissue. 

 In some mammaHan tumors (rhabelomyoma) the muscle 

 cells are striated. In others (leiomyoma) they are un- 

 striated. So far as we know no tumors of the first group 

 have been described in fowls, but Tyzzer and Ordway ^ have 

 described a tumor in the mesentery of a domestic fowl which 

 was composed of typical smooth muscle fibers. 



At present we know of no description of tumors of the 

 fourth or nervous tissue group in the domestic fowl. Such 

 tumors, however, very likely exist. 



We have seen that there are many different kinds of new 

 tissue growths or neoplasms in the domestic fowl. Some of 

 these are benign and some malignant. We are entirely 

 ignorant of the cause of these growths when they spontane- 

 ously occur in fowls. Only three out of thirty different 

 tumors tested by Rous, Murphy and Tytler ^ can be repro- 

 duced in another individual. None of these tumors has been 

 transmitted to healthy fowls kept with those which have 

 developed it. The work of Rous and others has, however, 

 shown that particular neoplasms have specific causative 

 agents which in some cases can be separated from the neo- 

 plasm. In the cases studied this was apparently a living 

 virus, although if organisms are present they are ultramicro- 

 scopic and able to pass through a Berkfeld filter impermeable 

 to Bacillus fluorescens liquefaciens. 



The work of Rous has shown that many birds possess a 

 perfect natural immunity to these tumors and that in general 

 young, vigorous birds are most susceptible. Funk ^ has shown 

 that birds stunted by a deficient diet are less susceptible than 



' Loc. cit. 



2 Loc. cii. 



'Funk, Casimir., "Studies on Growth: The Influence of Diet 

 on Growth, Normal and Malignant." Veterinary Journal, Vol. 21, 

 N. S., pp. 126-132, 1914. (Reprinted from Lancet.) 

 r 



