Effects of skeletal change on muscle pattern formation.

The skeletal influence on muscle development was investigated taking an experimental and comparative approach. Chick hindlimbs of HH stage 23 were operated in order to induce the development of an enlarged, elongated fibula - similar to the condition in bird ancestors. The skeletal change resulted in a series of secondary alterations in the later-forming zeugopod muscles. By comparing the results to the zeugopod musculature in other birds and in reptiles it could be shown that changes did not occur at random but resembled the patterns of interspecific variation and ancestral arrangement. It is concluded that skeletal elements influence muscle morphogenesis during a certain period of development. A classification of four phases in muscle development is presented: formation of the premuscular masses; individuation of separate muscles; final shaping and insertion; development maintenance and growth. The factors acting on each level are discussed and it is suggested that skeletal influence becomes effective in phase 3. The results also emphasize the role of development in evolution. The experimental reestablishment of an ancestral epigenetic condition within a developing organ system resulted in ancestral features of subsequently forming characters. This suggests that modification of epigenetic control of gene expression may be an important mechanism in evolutionary change.