Angiogenic response elicited by tumor masses growing in the cremaster muscle of athymic nude rats. The cremaster muscle of athymic male nude rats were inoculated with 5 × 104 human T24 bladder carcinoma cells. Ten days after tumor cell inoculation rats were sacrificed and mounted for photographic documentation. Concurrent with tumor growth we observed recruitment of new blood vessels that grew in a concentric pattern. Peri-tumoral edema characteristic of tumor growth can be observed. A. 1× magnification of the cremaster muscle bearing tumors on both sides of the principal artery, B. 2× magnification of left tumor, C. 2× magnification of the right tumor.