18 MASS. EXPERIMENT STATION BULLETIN 391 



the later stages of regression of implants of a transplantable hmphoid tumor 

 of the chicken (Olson 22). 



The results of the study seem to indicate that lymphocytoma is a systemic 

 disease affecting the widely distributed lymphoid cell system. The histogenesis 

 of lymphocytoma, therefore, does not conform with the commonly accepted 

 theory of histogenesis of neoplasia in which it is believed that tumors arise as the 

 result of a single cell or localized focus of cells assuming a state of neoplasia. In 

 spite of this feature, the invasiveness and apparently purposeless progressive 

 growth of lymphocytoma seem sufficient to characterize the disease as a neo- 

 plasm. Lymphoid tissue capable of becoming transformed into the malignant 

 cells of a lymphocytoma probably exists in nearly every part of the body, yet 

 certain organs were more often affected with the disease than others. These 

 organs are not necessarily those rich in lymphoid tissue, for the bone marrow, 

 thymus, and bursa of Fabricius in which such tissue is abundant were not found 

 affected as often as other organs relatively deficient in this respect. It is reason- 

 able to believe that the lymphoid tissue of such organs as the liver, spleen, kidney, 

 and gonad, in which lymphocytoma was more commonly found, is more sus- 

 ceptible to the action of the hypothetical causative factor or factors of lympho- 

 cytoma. This susceptibility may be due to some particular functional state of 

 the lymphoid tissue in those organs in which the disease occurs. 



Myelocytoma 



PentimaUi (25) described two cases of this disease and applied to it the term 

 "myelocytoma." Mathews (17) later gave an excellent description of the gross 

 and microscopic characteristics of 37 spontaneous cases to which he applied the 

 term "leukochloroma," adopted from a similar tumor of man which often has a 

 green color that is lacking in myelocytoma of the chicken. This fact together 

 with the prior application of the term "myelocytoma" would seem to be ample 

 justification for the use of the latter term. Twenty cases of myelocytoma were 

 encountered in this collection of neoplastic disease of the chicken. These cases 

 fit into the descriptions given by Mathews and, therefore, only a dursor\- resume 

 is necessary. 



The average age at necrops}- of 8 birds that died was 35.2 weeks, with the 

 oldest 55 weeks and the youngest 16 weeks. The average age of 12 birds that 

 were killed for examination was 32.7 weeks, with a maximum of 56 and a minimum 

 of 5 weeks. The age of birds with myelocytoma was more nearly like that of 

 birds with lymphocytoma than that of birds with leukosis (Tables 3, 11, 13). 

 The sex distribution was 15 females and 5 males, or a ratio of 3 to 1. Two cases 

 of myelocytoma were found to be associated with leukosis and are discussed in 

 the section dealing with leukosis. One case was found to be associated with 

 embryonal nephroma, and one with fowl paralysis. 



The liver was involved with the tumor in 16 instances. The amount of in- 

 filtration varied considerably but in general was moderate and usually confined 

 to the periportal areas. In two cases there was a rather marked hyperplasia 

 of lymphoid elements in the periporta' areas in addition to infiltration with neo- 

 plastic myelocytes. Since myeloid metaplasia may, occur in the periportal areas 

 of the liver in chickens without myelocytoma, it becomes necessary to establish 

 an arbitrary standard for the diagnosis of myelocytoma in the liver. The stand- 

 ard set in this work was that the myelocytes of a myelocytoma were to be at the 

 same stage of development; that is, there should be no evidence of transitional 

 forms from the lymphoid elements of the periportal areas to mature granulocytes. 

 When such a condition was found, the process was regarded as myeloid meta- 

 plasia and not myelocytoma. Myeloid metaplasia may be noted frequently 



