Let f : (C n , 0) → (C n+1 , 0) be a corank 1 finitely determined map germ. For a generic linear form p : (C n+1 , 0) → (C, 0) we denote by g : (C n−1 , 0) → (C n , 0) the transverse slice of f with respect to p. We prove that the sum of the image Milnor numbers µI (f ) + µI (g) is equal to the number of critical points of the stratified Morse function p|X s : Xs → C, where Xs is the disentanglement of f (i.e., the image of a stabilisation fs of f ).2000 Mathematics Subject Classification. Primary 32S30; Secondary 32S05, 58K40.