Factorisation of conformal maps on finitely connected domains

Let $U$ be a multiply connected domain of the Riemann sphere $\widehat{C}$ whose complement $\widehat{C}\setminus U$ has $N<\mathrm{\infty }$ components. We show that every conformal map on $U$ can be written as a composition of $N$ maps conformal on simply connected domains. This improves on a recent result of D.E. Marshall, but our proof uses different ideas, and involves the uniformisation theorem for Riemann surfaces.