A theoretical model is presented for fusion mechanisms in massive systems, where the fusion process is divided into two steps; an approaching phase up to the contact of two incident ions and shape evolutions from the amalgamated configuration to the spherical shape. Results of the first step provide initial values for the second step. A qualitative explanation and a simple expression of the extra-push energy are obtained with an inverted parabola approximating the conditional saddle. Results obtained with more realistic two-dimensional model for shape evolutions are compared very well with the available data for ⁴⁸Ca + actinide target systems. Remarks are given for synthesis of the superheavy elements.

PACS numbers: 24.60.--k, 25.60.Pj, 25.70.--z, 25.70.Jj