854 THE CARCINOGENIC STIMULUS II 



The recent reports of Gross (1957) and Stewart et al., (1957) indicate that the 

 activity of the leukemia and parotid tumor agents can be enhanced by frequent 

 transfer in mice or in cuhure in vitro. Schwartz et al. (1957) have reported on the 



Fig. 42. Leukemic mice. Cervical, inguinal, axillary and mesenteric lymp nodes (i) and 



spleen are enlarged in both specimens. In the one on the left, the thymus (th) is very large 



also. 



Fig. 43. Lymphomatous masses in the liver of a relatively old C3H mouse of a low-leukemia 



strain. Lymphomatous disease relatively infrequently appears spontaneously in these mice. 



If newborns are injected with leukemic extracts then leukemia appears early in life, and the 



incidence is probably higher than occurs spontaneously. 



Fig. 44. Smear of peripheral blood of a mouse with lymphocytic leukemia. All cells are 



lymiphoid. If these cells are inoculated into a genetically related animal they survive and 



proliferate giving rise to leukemia. X 250 



Fig, 45.- Section of lung of leukemic mouse. The large blood vessel in the center of the field 



is surrounded by a thick collar of leukemic lymphocytes (i i). X 70 



