[en] The postulated spur model is scrutinized by reference to μSF and MSR data in a variety of media. Specific arguments are given against the spur model when applied to: water and aqueous solutions of electron scavengers; low dialectric media such as hydrocarbons and liquid argon-and particularly the contrast between the muonium yield and the radiolysis free-ion yield; CC1₄ and its mixtures with hydrocarbons; and the absence of an effect of an external electric field. Comments are made regarding the timescale of muonium formation and on important comparisons with positronium formation. It is suggested that an expanding track model may account for the missing polarization which is observed in liquids but not in solids. (orig.)