  A combination of cellular contractility and active phase separation in
cell-matrix composites is thought to be an enabler of spatiotemporal patterning
in multicellular tissues across scales, from somitogenesis to cartilage
condensation. To characterize these phenomena, we provide a general theory that
incorporates active cellular contractility into the classical
Cahn--Hilliard-Larch{\'e} model for phase separation in passive viscoelastic
solids. We investigate the dynamics of phase separation in this model and show
how a homogeneous mixture can be destabilized by activity via either a
pitchfork or Hopf bifurcation, resulting in stable phase separation and/or
traveling waves. Numerical simulations of the full equations allow us to track
the evolution of the resulting self-organized patterns, in both periodic and
mechanically constrained domains, and in different geometries. Altogether, our
study underscores the importance of integrating both cellular activity and
mechanical phase separation in understanding patterning in soft, active
biosolids, and might explain previous experimental observations of cartilage
condensation in both in-vivo and in-vitro settings.
