1 M SQUARE FRAGMENTS OF MAMMARY TUMORS FROM WILD TYPE (MET-1) OR MUTATED (Db-7) PYV-MT (POLYOMA VIRUS MIDDLE T) WERE TRANSPLANTED INTO THE MAMMARY FAT PAD OR IN THE SUBCUTANEOUS TISSUE OF NUDE MICE. THE TUMORS WERE HIGHLY VASCULAR AND GREW RAPIDLY (NEXT SLIDE).