  (Abriged) At present, there are two scenarios for the formation of massive
stars: 1) The accretion scenario and 2) The coalescence scenario, which implies
the merging of intermediate mass stars. We examine here some properties of the
first one. We calculate three different sets of birthlines, i.e. tracks
followed by a continuously accreting star. First, three models with a constant
accretion rate ($\dot{M}_{\rm{accr}}$ = $10^{-6}$, $10^{-5}$, $10^{-4}$
M$_{\odot}$ yr$^{-1}$). Then several birthlines following the accretion models
of Bernasconi and Maeder (\cite{BM96}), which have $\dot{M}_{\rm{accr}}$
increasing only slightly with mass. Finally we calculate several birthlines for
which $\dot{M}_{accr} = \dot{M}_{\mathrm{ref}} ({\frac{M}{M_{\odot}}})
^{\phi}$, with values of $\phi$ equal to 0.5, 1.0 and 1.5 and also for
different values of $\dot{M}_{\mathrm{ref}}$. The best fit to the observations
of PMS stars in the HR diagram is achieved for $\phi$ between 1.0 or 1.5 and
for $\dot{M}_{\mathrm{ref}} \simeq 10^{-5}$ M$_{\odot}$ yr$^{-1}$.
Considerations on the lifetimes favour values of $\phi$ equal to 1.5. These
accretion rates do well correspond to those derived from radio and IR
observations of mass outflows. We emphasize the importance of the accretion
scenario for shaping the IMF, and in particular for determining the upper mass
limit of stars. In the accretion scenario, this upper mass limit will be given
by the mass for which the accretion rate is such that the accretion induced
shock luminosity is of the order of the Eddington luminosity.
