On the Singular Behaviour of Structure Functions at Low X

We discuss the phenomenological extraction of the leading $ j$-plane singularity from singlet structure functions $ F_s $ measured at small $ x. $ Using a saddle-point method we show that $ {\rm d ln} F_s /{\rm d ln}{1 \over x} $ is a suitable observable for this purpose in the region $x \le 10^{-2}.$ As an application, we confront and distinguish in a model-independent way structure function parametrizations coming from two different QCD evolution equations, namely the Lipatov (or BFKL) equation and the Gribov-Lipatov -Altarelli-Parisi (or GLAP) equation. Recent results at HERA are discussed in this framework.