  The study of the very young open cluster NGC 6231 clearly shows the presence
of a mass segregation for the most massive stars. These observations, combined
with those concerning other young objects and very recent numerical
simulations, strongly support the hypothesis of an initial origin for the mass
segregation of the most massive stars. These results led to the conclusion that
massive stars form near the center of clusters. They are strong constraints for
scenarii of star and stellar cluster formation.
