  We present a new simple functional form to model the evolution of a spherical
mass distribution in a cosmological context. Two parameters control the growth
of the system and this is modelled using a redshift dependent exponential for
the scale mass and scale radius. In this new model, systems form inside out and
the mass of a given shell can be made to never decrease, as generally expected.
This feature makes it more suitable for studying the smooth growth of galactic
potentials or cosmological halos than other parametrizations often used in the
literature. This is further confirmed through a comparison to the growth of
dark matter halos in the Aquarius simulations.
