  We present Plateau de Bure interferometer observations obtained in continuum
at 1.3 and 3.5 mm towards the six most massive and young (IR-quiet) dense cores
in Cygnus X. Located at only 1.7 kpc, the Cygnus X region offers the
opportunity of reaching small enough scales (of the order of 1700 AU at 1.3 mm)
to separate individual collapsing objects. The cores are sub-fragmented with a
total of 23 fragments inside 5 cores. Only the most compact core, CygX-N63,
could actually be a single massive protostar with an envelope mass as large as
60 Msun. The fragments in the other cores have sizes and separations similar to
low-mass pre-stellar and proto-stellar condensations in nearby protoclusters,
and are probably of the same nature. A total of 9 out of these 23 protostellar
objects are found to be probable precursors of OB stars with envelope masses
ranging from 6 to 23 Msun. The level of fragmentation is globally higher than
in the turbulence regulated, monolithic collapse scenario, but is not as high
as expected in a pure gravo-turbulent scenario where the distribution of mass
is dominated by low-mass protostars/stars. Here, the fractions of the total
core masses in the high-mass fragments are reaching values as high as 28, 44,
and 100 % in CygX-N12, CygX-N53, and CygX-N63, respectively, much higher than
what an IMF-like mass distribution would predict. The increase of the
fragmentation efficiency as a function of density in the cores is proposed to
be due to the increasing importance of self-gravity leading to gravitational
collapse at the scale of the dense cores. At the same time, the cores tend to
fragment into a few massive protostars within their central regions. We are
therefore probably witnessing here the primordial mass segregation of clusters
in formation.
