Histology and Growth Characteristics of Tumors

weeks from the time the pellet was removed. At two-week intervals animals from the control group and from the test groups were sacrificed and the tumors removed. Only those tumors classed as adenocarcinomas on initial bi opsy have been included in this study. Fibroadenomas (which do not regress on removal of the estrone pellet) and the few carcinomas which contained large amounts of connective tis sue, or which showed areas of hemorrhage, have been elimi nated from the study. The control group contained 7 animals bearing a totalof 31 tumors considered suitable for study, and the four experimental groups contained S rats bearing 21 tu mors, 8 rats bearing 34 tumors, 7 rats bearing 29 tumors, and 6 rats bearing 25 tumors respectively. Immediately following excision, the tumors were divided into several portions, fixed in Susa fixative, and embedded in wax. Several random sections were cut from each block at 4 j.z and stained with hematoxylin and eosin or with iron hema toxylin. Where total regression of tumor had occurred, as at eight weeks, the entire mammary gland was removed, serial blocks made, and every tenth section throughout the thickness of each block examined.