SPONTANEOUS NEOPLASMS IN MICE 215 



called aleukemia (pseudo-leukemia). Without blood smears, it is not easy to 

 classify these tumors as to whether they are leukemic or aleukemic. Some 

 workers (48) have drawn their conclusions from a study of the large blood 

 vessels of the liver, lungs and kidneys. Tissue imprints with special stains 

 have been very valuable in determining the types of abnormal cells present in 

 the tissues {t,^). Generally, the greater the number of tumor cells in the 

 blood, the less the lymph nodes are enlarged and vice versa. 



Most authors agree that there are no benign tumors produced bv either 

 lymphocytes, myeloid cells or monocytes. The non-neoplastic condition 

 which has caused the most confusion is probably non-malignant extramedul- 

 lar)' myelopoiesis. This condition is very common in the spleen of older 

 mice of some stocks (3). In extramedullary blood forming foci all the ele- 

 ments of the normal marrow are usually present. The granulopoietic 

 elements most often predominate over the er>'thropoietic and megakaryo- 

 cytic elements. This condition is found most frequently in the spleen and 

 liver. The sites usually involved by extramedullary myelopoiesis are 

 similar to those in cases of myeloid leukemia. In the former all stages of 

 development of myeloid cells are present, while in the latter most myeloid 

 cells are immature. Additional information on the dilTerences between 

 these two conditions can be found under myeloid tumors. 



Lymphocyte Tumors (Lymphoblastoma) 



The most commonly observed tumors of the lymphatic system are those 

 of the lymphocytes (Fig. 117). They appear first in the nodules of the 

 lymph nodes and in the Malpighian bodies of the spleen. The primar\' foci 

 increase in size, due to proliferation of the lymphocytes, and progress until 

 they obliterate the normal architecture of the lymph nodes and spleen, 

 leaving only uniform masses of lymphocytes. These tumors always invade 

 the lymph node capsules (48). Due to the extent of the lymphatic sys- 

 tem, infiltration of adjacent tissues is difticult to differentiate from true 

 metastases. 



Lymphocytoma. — This shows fairly uniform cells of the lymphocyte 

 type; however, they are larger than normal cells. Usually they belong to 

 the large lymphocyte variety. Mitotic figures are often numerous (Fig. 

 117). The liver, lungs and kidneys are the organs most often invaded. In 

 the liver the periportal foci are first involved, in the lungs perivascular 

 infiltration is most marked and in the kidney the infiltration extends inward 

 from the hilus. This tumor mav be leukemic or aleukemic, focal or more or 



