According to the consequences of the density wave theory of spiral structure a shock wave is triggered in the interstellar gas leading to the formation of new stars (Roberts, 1969). The space and velocity distribution of the newly born stars is similar to those of the diffuse material which they have been formed from, i.e. they are strongly concentrated to the Galactic plane and have nearly circular velocities and small velocity dispersions. The velocity dispersions could increase in course of time by different stellar dinamical processes: encounters with large clouds of stars and gas (Spitzer and Schwarzschild, 1953; Julian, 1967; Barbanis and Woltjer, 1967), cooperative phenomena (Lynden-Bell, 1967), effect of non periodic orbits (Wielen, 1975), etc. If some of this processes has a time scale comparable with the time the star streeming needs between two consecutive passages of the density wave one expects an observable effect in the space distribution of A and late B type stars perpendicular to the Galactic plane. The Δt time difference between two consecutive passages equals 2.5×108 years near the sun. If the lifetime of stars is greater than this value their space distribution is a superposition of newly born stars with small velocity dispersion and stars born earlier and having greater velocity dispersion already. (Fig. 1.)