The working hypothesis ... is that the bacteriophage is either a definite stage in the cyclogeny of the bacterial species, or a functionally active particle accessory to one of these stages; and by the term "accessory" I mean possessing complementary or reciprocal biologic significance, such, for example, as the relation of sperm cell to ovum. With such a conception there is not any priority of significance in the relation between bacteriophagic corpuscle and the cell which it "attacks." Both elements are necesary components of a definite reproductive mechanism possessed by many, if not all, bacteria. This constitutes the nucleus of what I have termed my "homogamic theory" of bacteriophage action. PHILIP HADLEY