Studies on the HST (Hamazaki) virus isolated from Yoshida tumor.

is due to some external agent capable of sustained action, and the assumption that this agent is living and multiplying in the neoplasm appears to be the best explanation for its persistent effects. It is now known that many malignant tumors in birds and a few benign tumors in mammals are transmissible by means of cell-free filtrates, and there are good reasons for believing that the ac tive agents in these filtrates are viruses. The Yoshida tumor is an ascites tumor of the rat (18) ; it has been assumed to be derived from the serous cells of the peritoneum (4), from the stellate cells of the liver (14), or from monocytes (1). The possible virus etiology of the ascites tu mor was first investigated by Yoshida and his co workers, but their experiments led them to dis card this hypothesis. Kinoshita (9) and his asso ciates believed at first that this tumor was caused by a virus, because it could be transmitted by the inoculation of blood serum or urine from tumor bearing rats into healthy ones. Since the possi bility of free tumor cells in these fluids was not excluded, however, they discarded the virus the 017. Terada et al. (11) also objected to the virus theory on the basis of experiments showing the nontransmissible character of the ascites filtrate prepared by means of Berkefeld filters. Ishibashi (8) also discarded the possibility of virus etiology after the following experiments. He diluted tumor ascites 4†" 17times and centrifuged the resulting suspension at 3,000 r.p.m.' for 30†" 90 minutes. Droplets of the supernate were brought under the micromanipulator, observed to be strictly cell free, and injected into the peritoneal cavity of nor mal rats. With one exception, the results of the fifteen experiments were negative. The rapidity of the accumulation of ascites in the single posi tive case was similar to the rate of ascites forma tion in experiments where one or two tumor cells had been inoculated. Therefore, this author be lieved that there were no data to exclude the pos sibility of an accidental persistence of a single tumor cell in the material used for the inocula tion. Yasuda and Hasegawa (1@) injected the supernate of tumor ascites centrifuged at 3,000 r.p.m. into healthy rats with positive results, but found that when they injected a Seitz filtrate no ascites appeared, except in one castrated animal. These …