  The mass function of clumps observed in molecular clouds raises interesting
theoretical issues, especially in its relation to the stellar initial mass
function. We propose a statistical model of the mass function of prestellar
cores (CMF), formed in self-gravitating isothermal clouds at a given stage of
their evolution. The latter is characterized by the mass-density probability
distribution function ($\rho$-PDF), which is a power-law with slope $q$. The
variety of MCs is divided in ensembles according to the PDF slope and each
ensemble is represented by a single spherical cloud. The cores are considered
as elements of self-similar structure typical for fractal clouds and are
modeled by spherical objects populating each cloud shell. Our model assumes
relations between size, mass and density of the statistical cores. Out of them
a core mass-density relationship $\rho\propto m^x$ is derived where
$x=1/(1+q)$. We found that $q$ determines the existence or non-existence of a
threshold density for core collapse. The derived general CMF is a power law of
slope $-1$ while the CMF of gravitationally unstable cores has a slope $(-1 +
x/2)$, comparable with the slopes of the high-mass part of the stellar initial
mass function and of observational CMFs.
