When a particular mutagen identified by the ames test is injected into mice, it causes the appearance of many tumors, showing that this substance is carcinogenic. when cells from these tumors are injected into other mice not exposed to the mutagen, almost all of the new mice develop tumors. however, when mice carrying mutagen-induced tumors are mated to unexposed mice, virtually all of the progeny are tumor free. why can the tumor be transferred horizontally (by injecting cells) but not vertically (from one generation to the next)?