SPONTANEOUS NEOPLASMS IN MICE 175 



niary region. 'J'hc normal mammary glands have been full}- discussecJ 

 under the section on histology, but for convenience the distribution of the 

 mammae can be briefly reported here. There are five pairs of glands 

 arranged symmetrically along the ventral surface of the mouse (Fig. 89). 

 This rather extensive distribution of the mammary glands is referred to here 

 as the mammary line and its branches. 



Because of the accessibility to observation the subcutaneous tumors are 

 probably better known than those of other body parts, and since the major- 

 ity of tumors observed are in or near the mammary region, they must be 

 studied histologically to determine their true nature. 



Mammary region tumors have been divided into two main groups in an 

 attempt to include all the types of tumors which occur in the region of the 

 mammary line and its branches. First are those tumors which originate 

 from the mammary gland proper while the second group comprises all other 

 tumors in this same location, but not arising from the mammary gland or its 

 supporting stroma. This grouping is intended to cover the masses 

 which, by gross observation, might be mistaken for tumors of mammary 

 gland origin as well as those which arise from the gland itself. 



Classification of Tumors in or Near the Mammary Glands 



I. Tumors originating from the mammary gland proper. 



A. Benign tumors. 



1. Simple adenoma. 



2. Polylocular cyst adenoma. 



3. Papillary cyst adenoma. 



4. Fibro-adenoma (adenofibroma). 



B. Malignant tumors. 



1. Adenocarcinoma — definite evidence of mammary- gland origin 

 predominates the histological picture. 



a. Simple adenocarcinoma. 



h. Adenocarcinoma (variable type). 



c. Papillary cyst adenocarcinoma. 



d. Intracanalicular adenocarcinoma. 



e. Macroglandular adenocarcinoma. 



2. Carcinoma simplex — little evidence of definite gland formation. 



a. Round cell or medullary. 



b. Spindle cell. 



3. Carcinosarcoma — originating from both the mammary gland 

 epithelium and the stromal connective tissue. 



4. Fibrosarcoma — originating from the mammary gland stroma. 



