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1. Intra-axonal organelles were detected by darkfield and Nomarski microscopy in isolated myelinated nerve fibres from Xenopus laevis. Nerve fibres from the 8th spinal roots, the sciatic nerve, and identified motor and sensory axons from other hind limb nerves were used. The movement of the organelles was recorded either on motion picture film or by noting the times at which they crossed the lines of an ocular grid.

2. Three groups of organelles were detected in all fibres. A group of particles with round profiles 0·2-0·5 µm in diameter moved somatopetally. Another group of round particles moved somatofugally. The ratio of the number of somatopetally travelling particles to the number of somatofugally travelling particles was about 10:1. The third group of organelles consisted of rod-shaped bodies about 0·2-0·3 µm in diameter and 1-8 µm in length; these were usually stationary.

3. All the round particles appeared to move independently of each other with a saltatory motion. The somatopetally and somatofugally travelling particles had statistically different mean velocities of 0·98 and 1·32 µm/sec respectively.

4. Round particles often crossed the node of Ranvier with no appreciable change in velocity. Some, however, were temporarily arrested at the entrance to the node.

5. While the rod-shaped organelles were usually stationary, they occasionally moved rapidly lengthwise for distances of up to 10 µm. Rarely a rod-shaped organelle exhibited a continuous saltatory motion.

6. Round particles often travelled in either direction along the edge of rod-shaped organelles. One rod was observed to move along the path previously taken by a round particle.

7. The findings are discussed with respect to (a) the normality of the preparations, (b) the numbers of particles travelling in each direction, (c) the nature of the organelles, and (d) the mechanisms underlying the motion.

8. We suggest that particles move along microtubules which have specific directionalities and particle affinities. The microtubules are in bundles and are closely associated with rod-shaped mitochondria.

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