233 nerves were fixed in ice-cold, buffered, I o/osmium tetroxide prepared in the respective physiological solution, and embedded in methacrylate. Thin sections were made in a LKB Ultratome and observed in a Siemens Elmiskop I. The results are shown in Figs. 1-4. As observed in the electron micrographs the thorium dioxide particles are found in (a) the Schwann cell channels and the axon-Schwann-cell space of the squid giant nerve fibres, Fig. r; (b) the mesaxon gap and the axon-Schwann-cell space of the medium and small size unmyelinated fibres of the squid, Fig. 2; (c) the mesaxon gap and the axon-Schwann-cell space of the unmyelinated fibres of the flog, Fig. 3; and (d) the Ranvier-node gap of the frog myelinated fibre, Fig. 4. The restriction offered by the basement membrane of the squid giant fibre to the diffusion of thorium dioxide is clearly shown in Fig. I.
Besides confirming our earlier observations on the structure and permeability of the Schwann cell channels and the axon-Schwann-cell space of the giant nerve fibre of the squid, the present results permit us to generalize on the basic aspects of those findings to the mesaxon gaps and the axon-Schwann-cell space of other unmyelinated fibres and the Ranvier-node gaps of the myelinated nerve fibres.