Some Remarks on the OB Associations in Our Neighbourhood

It is pointed out that the Orion Arm offers, more than the more remote spiral features in or beyond the Galaxy, the opportunity to compute and relate the process of star birth in adjacent domains and to approach the problem concerning the manner in which star formation spreads within a spiral arm. Currently available data on the Orion Arm within 1500 pc provide no evidence for progression of star formation in one particular direction. However, this conclusion is of a provisional nature because data for more distant OB associations are still rather incomplete. Attention is given to the problem of the initial sizes of the subgroups of OB associations resulting from the formation process.