An initial stage of the transition from 2D to 3D growth has been analyzed in the framework of a simple model of two-level pyramids consisting of 2D islands. We show that the characteristic time of the transition to 3D growth may depend non-monotonically on the deposition temperature. At high T the formation of a 3D island is ensured by the flux of atoms detaching from the edge of a 2D island to its surface, whereas at low T, by the flux of adatoms crossing the 2D island edge and passing the kink. The later mechanism takes place when migration of adatoms along the island edges is slow and there is an Echrlich-Schwoebel barrier and a trap for adatoms on the island surface.

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