NATU IR E May 22, 1954 v. a results of Buchthal et al.,?, The natural conclusion, that the material which gives the A-bands their high refractive index and also their birefringence is in the form of submicroscopic rods of definite length, was put forward by Krause, and receives strong support from the observations reported here. The identification of this material as myosinº, and the existence of filaments (presumably actin) extending through the I-bands and into the adjacent A-bands, as shown in many electron microscope studies, makes very attractive the hypothesis that during contraction the actin filaments are drawn into the A-bands, between the rodlets of myosin.(This point of view was reached independently by ourselves and by HE Huxley and Jean Hanson in the summer of 1953. It has already been mentioned by one of those authorsº and is further discussed by them in the accompanying article.)

If a relative force between actin and myosin is generated at each of a series of points in the region of overlap in each sarcomere, then the tension per filament should be proportional to the number of these points, and therefore to the width of this zone of overlap. If the myosin rods are I-5 p. long and the actin filaments 2-0 p, the isometric tetanus tension should fall linearly as the fibre is stretched over the