Following destruction of the right anterior member bud on four-day old chick embryos, a study was made of right and left pectoral muscle growth on serial sections stained by ordinary methods. Certain regions were also examined in the electron microscope. Mitotic activity was studied autoradiographically following the injection of thymidine. Early, frank changes in the pectoral muscle on the amputation side were connected with growth, differentiation and organogenesis: regressive processes were also observed.