226 



BIOLOGY OF THE LABORATORY MOUSE 



grading over into carcinoma simplex tumor cells which are invading the 

 uterine wall. 



Of the non-epithelial tumors fibrosarcoma is the most common tumor of 

 the uterus (Fig. 124B). Quite a number of cases have been seen. This, 

 however, does not represent a high incidence in any of the pure stocks or 





mu. bn. 



Fig. 123. — Teratoma of the ovary (X200). bn., bone; cart., cartilage; gl., gland; mu., 



striated muscle. 



their hybrids. Of the pure stocks it is probably most common in the dba. 

 It is, however, seen in the C57 black mice. Most of our cases have devel- 

 oped in crosses between these two stocks or in hybrids between C57 black 

 and A albino. 



With this tumor the uterus is greatly enlarged, firm and friable. The 

 enlargement is usually bilateral and these tumors are not multiple as is the 

 case of the fibroid tumors in the human. 



Histologically this tumor is composed of small, closely packed, short 

 spindle cells with little stroma. The cells are arranged in an irregular inter- 

 lacing pattern of whorls (Fig. 124B). Mitotic figures are not common but 



