An electron microscope study of the development of the somatic muscle of the chick embryo

Abstract Somites 16 through 18 were obtained from chick embryos from 36 hours to 15 days incubation, and prepared for both light and electron microscopy. It was found that the myoblasts in the early stages increase in size and become elongated. Myofilaments of two types are first discernible in the peripheral cytoplasm in the 32 somite stage. Those of one type are about 100 Å in diameter and are probably myosin, while the others are approximately 70 Å and are believed to be actin. These filaments are organized into bundles (myofibrils) which are from the first striated. Throughout later development recognizable myofibrils are present; alongside are found numerous myofilament bundles in all stages of development indicating continuous myofibril formation. Smooth‐surfaced vesicles and tubules become the main constituent of the sarcoplasmic reticulum. Free ribosomes numerous at first, take up positions around the developing filament bundles and decrease in number as the myofilaments are formed. At the time of appearance of the first myofilaments, glycogen granules become visible in the sarcoplasm and increase rapidly in number. The mitochondria become arranged between the developing myofibrils.