44 



MASS. EXPERIMENT STATION BULLETIN 39i 



sarcoma. The rate of incidence of these tumors was essentially similar in col- 

 lections A and B. Leiomyoma, epithelioblastoma, and carcinosarcoma seemed 

 to be relatively more common in chickens from the source of collection B and 

 may be regarded as types of neoplasia associated with advanced age (the average 

 ages of birds in which such tumors were found were 66.5, 67.1, and 114.1 weeks, 

 respectively, Table 22). The greater incidence of these tumors in collection B is 

 probably due to the different proportion of old birds examined. More than twice 

 as many birds over 12 months of age were examined from the source of collection 

 B (237 birds) as from the source of collection A (1 10 birds). Embryonal nephroma 

 appeared to be more common in birds of collection A, but the addition of one or 

 two more cases to collection B would have raised the incidence rate to a level 

 comparable with that of collection A. 



Therefore, collections A and B were essentialK' similar with respect to the 

 incidence of the commoner forms of neoplasia. 



In considering all forms of neoplasia, lymphocytoma was the most common 

 (55.5 percent of 384 tumors studied). Although none of the other types of neo- 

 plasia were of striking frequency, six other varieties (leiomyoma, embryonal 

 nephroma, myelocytoma, leukosis, epithelioblastoma, and fibrosarcoma) ac- 

 counted for 3i percent of the tumors. 



Table 21. — Distribution of Chickens over Six Weeks of Age Examined 



FOR Disease. Arranged x-Xccording to Age, Breed, Sex, and Quarter of 



Year When Necropsied. 



Collections A and B of Neoplasms were obtained from these birds. 



*No chickens from Collection B are represented in these groups. 



Breed 



Neoplasia was found in only three breeds to an extent sufficient to warrant 

 detailed study of the data. The difficulty of dealing with small figures again 

 presents itself. A preliminary examination of the data in Tables 21 and 22 in- 

 dicated that comparisons should be made of the incidence of lymphocytoma, 

 myelocytoma, fibrosarcoma, and leukosis. The comparison was drawn and the 



