1. Frog and toad sartorius muscles were soaked in ferritin suspensions and then fixed and prepared for electron microscopy. Ferritin particles were counted in micrographs of regions where fibril striations were in good register, and the number of particles plotted according to position along the sarcomeres.2. The distribution of ferritin particles in both frog and toad muscle sarcomeres could be accounted for by a single peak centred at the Z-line, with a total width of about 0·4 μ.3. The peak in toad fibres was slightly broader than that in frog fibres, and this was due to more frequent branching of the T-system in the toad. In both frog and toad sartorius muscles branching increases the total quantity of T-system by around 30%.4. No peak was found in the ferritin distribution which could account for the peak of albumin distribution observed near the A—I boundary of toad sartorius muscles by Hill (1964).
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