Thin sections of striated muscle from frog (sartorius), rabbit (psoas), rat (heart), and fly (flight muscle), and of smooth muscle from clam (adductor) have been obtained using a new microtome. Electron micrographs of them are presented. The sections are sufficiently thin to achieve a resolution of 30 to 40 A. In fly flight muscle, the myofibrils are distinct and well separated morphological units. In frog and rabbit muscle the myofibrils appear to be so closely packed under normal conditions that their identity as separate units is almost lost. In all types of striated muscle examined, it was found that the filaments are arranged within the myofibrils in a continuous and highly regular hexagonal array. The diameter of the filaments in embedded, sectioned muscle appears to be significantly less than that observed in dried shadowed material. The 400 A axial period, observed in frog and rabbit muscle, and in rat heart muscle, was found to extend across the interstitial material between the filaments.

The significance of these findings is discussed.

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