Certain axons in the abdominal roots and nerve cord of crayfish contain a system of regularly spaced, parallel transverse septa with a periodicity of about 2 µ. Each septum is composed of two roughly parallel membranes, separated by a gap of 150–400 A. The two membranes are frequently fenestrated by pores 550–2000 A in diameter, each occupied by a microtubule. Filaments are occasionally seen bridging the gap between the microtubule and the edge of the pore. The membranes of the septa are continuous with longitudinal membranous tubules. In small- and medium-sized axons the septa are continuous across the axon, while in large axons they seem to be intact only at the periphery as annuli. It is suggested that such structures be called "fenestrated septa." With horseradish peroxidase as a tracer, no communication between the septal lumen and the periaxonal space was found.

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