Macromolecular arrangement within muscle.

Electron microscopy has already given much information about the macromolecular components of striated muscle (1, 4). Thus it is shown that myofibrils of teased muscle are ribbons composed of parallel arrays of filaments associated with an amount of seemingly amorphous material that is great,est in the anisotropic regions. For the present work, strips of psoas muscle separated from a rabbit at death were tied in situ to strips of wood at their resting length, then cut out and immediately fixed in formalin. Pieces of this fixed muscle were dohydrated by passage through alcohols, embedded in methacrylate, and thinly sectioned for electron microscopy by the procedures recently outlined by Neumann, Borysko and Swerdlow (2) and finally shadowed with goldManganin. In favorable instances an astonishingly regular macromolecular arrangement can be seen in these sections. dt a moderate magnification a section of a fiber cut nearly at right angles to the long axis will appear as in Fig. 1. The macromolecular filaments constituting the myofibrillar blocks are seen almost end-on as either dots or short rods. Their diameters correspond to those of the fila-