Motor pool positions for individual leg muscles were mapped in hatched chicks using intramuscular injections of HRP. Several leg muscles were also mapped in stage 38 (12-day) embryos. The results indicate that an adult map is formed by stage 38.

The adult motor pool map can be viewed as being composed of two maps, one for muscles derived from the embryonic ventral muscle mass, the other for muscles derived from the dorsal mass. Each of these maps is a continuous, although distorted, representation of muscle precursor position on the original sheets of dorsal and ventral muscle mass. The developmental implications of these findings are discussed.