Dynamics of soleplate formation. 
Immunohistochemical techniques with anti-desmin, anti-acetylcholine receptor and anti-fibronectin antisera and autohistoradiography were used to determine the dynamics of neuromuscular synaptogenesis.  Fast twitching muscles were taken from chick embryos at 5 to 14 days of incubation. " Primitive eminences" at terminal arborizations of motor neurons were composed of Karnowsky positive, anti-desmin and anti-acetylcholine receptor positive cells which contained sites bound to alpha-bungarotoxin.  These cells, characterized as myoblasts, fused with the myotubes during formation of neuromuscular junctions in the sites of contact with terminal arborizations of motor neurons.  Their nuclei and cytoplasmic organelles become the nuclei and organelles in the soleplate.