SPONTANEOUS NEOPLASMS IN MICE 



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glands are nearly the same size as normal mammary gland. ]\Iost of them 

 form irregularly shaped cysts, which vary greatly in extent and which derive 

 their outlines from the size, shape and number of papillary growths which 

 extend into them. 



Fibro-adenoma. — This tumor of the mammary gland usually has the 

 same type of gland-like arrangement seen in the simple adenoma. Histo- 



Large spindle cells 



Adenomatous foci 



Fig. q4. — Adenofibroma of the mammary gland (X200). 



logically, the stroma has the same type of wayy, thread-like connective 

 tissue cells with spindle-shaped nuclei around the adenomatous foci. The 

 difference between these adenomas lies for the most part in the two chief 

 characteristics of the hbro-adenoma ( Fig. 93) . First, that the stroma is more 

 abundant and makes up nearly as much of the bulk of the tumor as does the 

 adenomatous parts. Second, that there are strands and bundles of large, 

 closely packed connective tissue cells running in all directions throughout the 

 stroma. These large cells are narrow, long, tapering at the ends and have 

 centrally placed nuclei. The cytoplasm is abundant and uniform, taking a 



