STUDIES ON MUSCLE DIFFERENTIATION. III. IMMUNO-HISTOCHEMICAL IDENTIFICATION OF TROPOMYOSIN IN SKELETAL AND CARDIAC MUSCLES OF DEVELOPING CHICK EMBRYO.*,**

Specific identification of tropomyosin was succeeded in chick embryos by an application of immuno-histochemical techniques with the antisera against frog skeletal muscle tropomyosin. The first appearance of tropomyosin was in the central part of cervical somites of stage 14 embryo, and the area containing tropomyosin extended dorso-ventrally in later stages. In the heart primordium, tropomyosin was first detected in the presumptive epimyocardium at stage 8 embryos and was found to be concentrated in the epimyocardium in later embryos. The stages of the first appearance of tropomyosin in somites and presumptive myocardium corresponded with those of the first appearance of thin filaments in these organ primordia reported by other investigators. Tropomyosin in myogenic cells and in muscle fibers was localized in cell membrane or in cell peripheries. It was also distributed in a striated pattern which seemed to be due to a localization of tropomyosin in the I-bands.