  The non-linear evolution of a stratified perturbation in a three dimensional
expanding Universe is considered. A general Lagrangian scheme (Q model) is
introduced and numerical investigations are performed. The asymptotic
contraction of the core of the agglomeration is studied. A power-law scaling is
detected and an heuristic interpretation of the numerical findings is provided.
An asymptotic equation for the multi-stream velocity flow is derived and it is
shown to agree quantitatively with the dynamics of the Q model. The relation to
the adhesion model is discussed.
