THE GENETICS OF SPONTANEOUS TUMOR FORMATION 265 



ratios with which to test the exact genetic nature of the process of tumor 

 formation. 



There are, however, certain general factors about non-epithehal tumors 

 that differentiate their time, place and rate of origin from that of epithelial, 

 mammary or lung tumors. 



As, in the case of mammary or lung tumors, one must begin his investiga- 

 tion of the genetics of the incidence of non-epithelial tumors in inbred 

 material unless he wishes deliberately to handicap himself. 



Even in types of tumors which histologically are relatively consistent, as 

 the adenocarcinomas of the mammary gland or lung, there are modifying 

 and complicating influences which afifect the genetic processes. This is even 

 more applicable to the non-epithelial tumors so that, in this chapter, empha- 

 sis will be placed upon a study of a few inbred strains and their hybrids. 



In this connection a general statement contained in a recent paper by 

 Little, Murray and Cloudman (60) may be helpful. The authors, in describ- 

 ing the commoner types of non-epithelial tumors, state : 



" Tumors of lymph cells may occur wherever lymphatic tissue is present. 

 They generally are primary in the spleen or in the various mesenteric or other 

 peritoneal nodes. When they are thus situated the clinical symptoms are an 

 abdominal swelling, with hardening, frequent turgidity, ascites or general- 

 ized edema, and occasional asymmetry. Often by careful palpation the 

 enlargement of the spleen or the presence of other peritoneal nodules can be 

 detected. In some animals the peritoneal cavity may be grossly distended 

 by fluid. This fluid is of three types. It may be hemorrhagic, of the deep 

 color of venous blood; in other cases it is milky with a pink tinge; while in 

 still others it is clear and watery. As yet no consistent correlation between 

 any of the three types of fluid and any particular character of the lymphatic 

 tumor has been detected. 



"The next most frequent type of non-epithelial tumor is a reticulo-endo- 

 thelioma of the liver. Fluid within the peritoneal cavity is rare in associa- 

 tion with tumors of this type. Abdominal swelling occurs, however, due to 

 the enlargement of the liver. Naturally this swelling tends to be more 

 anterior in position than many of the masses in the lymphocyte tumor 

 group. 



" Fibrosarcomas are apt to occur in scattered sites. Those on the jaw or 

 leg or in the dorsal or lateral subcutaneous tissue are readily discernible as 

 hard, fixed nodules of firm texture. Those in the uterus are usually recog- 

 nizable by posterior distention of the abdomen and by the presence of an 

 irregular palpable nodule. 



