  Open clusters (OCs) are infrequent survivors of embedded clusters gestated in
molecular clouds. Up to now, little is known about the initial conditions for
the formation of OCs. Here, we studied this issue using high-precision
astrometric parameters provided by Gaia data release 3. The statistics show
that the peculiar motion velocities of OCs vary little from infancy to old age,
providing a remarkable opportunity to use OCs to trace their progenitors.
Adopting a dynamical method, we derived the masses of the progenitor clumps
where OCs were born, which have statistical characteristics comparable to
previously known results for clumps observed in the Galaxy. Moreover, the
masses of the progenitor clumps of OCs indicate they should be capable of
gestating massive O-type stars. In fact, after inspecting the observed OCs and
O-type stars, we found that there are many O-type stars in OCs. The destructive
stellar feedback from O-type stars may disintegrate the vast majority of
embedded clusters, and only those sufficiently dense ones can survive as OCs.
