  We analyze in detail the anomaly cancellation conditions for the strongly
coupled $E_8 \times E_8$ heterotic string introduced by Horava and Witten and
find new features compared to the ten-dimensional Green-Schwarz mechanism. We
project onto ten dimensions the corresponding Lagrangian of the zero-mode
fields. We find that it has a simple interpretation provided by the conjectured
heterotic string/fivebrane duality. The part which originates from
eleven-dimensions is naturally described in fivebrane language. We discuss
physical couplings and scales in four dimensions.
