Assay of frozen mouse mammary carcinoma for the mammary tumor milk agent.

cause and that this, in mammals as in birds, is a virus.― Mann (33—35)injected frozen spontaneous and transplanted mammary cancers and explained the development of tumors on the basis that the mam mary tumor milk agent-virus was liberated in its active form following freezing, since mammary tu mors arose at the site of injection of the suspended frozen tumor-mince. The tumors developed as readily in males as in female mice, but only when the frozen tissue was injected into the mammary gland region, because of what the author called “selective infectivity.― She found that, within limits, the longer the tissue was frozen, the more tumors developed, and the interval between injec tion and the appearance of the tumors was related to their rate of growth. Fast growing cancers pro duced growths within 10 days, while slow growing ones might be delayed for 7—1@weeks following the inoculation of their frozen suspensions. Mann (35) also demonstrated that mammary tumors could be produced by the injection of tumors which had been frozen and dried in vacuo (15), provided no thawing occurred before the material was dry. Gye, Begg, Mann, and Craigie (@9) reported that some tumors could withstand a succession of freezing and thawing, which they claimed com pletely excluded the persistence of tumor cells, but Mann (34) observed a progessive deterioration of activity of mammary tumors after a second thaw ing. Individual tumors differed, but in no instance did a tumor inactive after a second thawing pro duce tumors after a third. The exact age of the animals at the time of injec tion was not stated except that “Although all came