Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Medical Sciences


M.P. Rathbone


The purpose of the studies reported in this thesis was to obtain data concerning the embryonic origin and formation of the pectoralis major muscle of the chicken. This muscle is used extensively in studies on muscle development because it is large; readily available, and is composed almost entirely of one muscle fiber type. Moreover, it is the largest muscle to be affected by hereditary muscular dystrophy in the line of chickens afflicted with this disease. Information concerning its embryonic origin could be used for in vivo studies on the early development of both normal and dystrophic muscles.

Previous investigations into the embryonic origin od skeletal muscle in several classes of vertebrates have resulted in controversy. Some investigators have concluded that all skeletal muscles arise from the myotomal layer of the somites. Others have cited evidence to show that some muscles, including the pectoralis major muscle of the chick, are derived from the somatopleuric mesoderm adjacent to the somites.

In the present investigation, interspecific chimaeras have been used to study the problem. Whole somites, somite halves, or limb-buds were grafted from quail to chick embryos between 2 and 3 days in ovo. After further development, the chimaeras were fixed, embedded in paraffin, sectioned, and stained using the Feulgen reaction for chromatin. This procedure permitted the identification of those structures that were derived from the grafted quail tissue.

The observations in this study have led to the following conclusions:

The pectoralis major muscle arises from the dorsal halves of somites 16-21 of the 2-day in ovo chick embryo. These somites also give rise to all other wing and wing-associated muscles of the shoulder and thorax. Each somite plays a specific role in the development of these muscles. The cells that ultimately form the pectoralis and other brachial muscles migrate from the somites into the lateral mesoderm between 2 and 2.5 days in ovo. The myotomal layers of the somites do not appear until 2.5 days in ovo and do not contribute to the formation of the brachial muscles.

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