  We investigate the intrinsic stellar populations (estimated total numbers of
OB and pre-main-sequence stars down to 0.1 Mo) that are present in 17 massive
star-forming regions (MSFRs) surveyed by the MYStIX project. The study is based
on the catalog of >31,000 MYStIX Probable Complex Members with both
disk-bearing and disk-free populations, compensating for extinction,
nebulosity, and crowding effects. Correction for observational sensitivities is
made using the X-ray Luminosity Function (XLF) and the near-infrared Initial
Mass Function (IMF)--a correction that is often not made by infrared surveys of
young stars. The resulting maps of the projected structure of the young stellar
populations, in units of intrinsic stellar surface density, allow direct
comparison between different regions. Several regions have multiple dense
clumps, similar in size and density to the Orion Nebula Cluster. The highest
projected density of ~34,000 stars/pc^2 is found in the core of the RCW38
cluster. Histograms of surface density show different ranges of values in
different regions, supporting the conclusion of Bressert et al. (2010, B10)
that no universal surface-density threshold can distinguish between clustered
and distributed star-formation. However, a large component of the young stellar
population of MSFRs resides in dense environments of 200-10,000 stars/pc^2
(including within the nearby Orion molecular clouds), and we find that there is
no evidence for the B10 conclusion that such dense regions form an extreme
"tail" of the distribution. Tables of intrinsic populations for these regions
are used in our companion study of young cluster properties and evolution.
