Sections of the peripheral nerve-trunks of the metathoracic leg of the cockroach (Periplaneta americana) were studied with the electron microscope. Paraffin sections were also prepared and stained. Protargol succeeds in staining the nerve-fibres. Osmium tetroxide, a modified Weigert procedure, and Luxol fast blue stain the myelin sheaths, as does mercuric bromphenol blue, a protein stain. The axoplasm is relatively free of formed elements; it contains mitochondria. The myelin sheath, when present on the largest and also some smaller fibres, consists of about two or three loose over lapping processes of Schwann cells, covered by their plasma membranes, enclosing lipid-like droplets and having a beaded appearance. Between the nerve-fibres in the nerve-trunk is Schwann-cell cytoplasm, which arises from Schwann cells that surround the whole nerve-trunk. The same fold of Schwann-cell membrane may enter into the formation of the myelin sheath around more than one nerve-fibre. Several small non-myelinated fibres, which may be as small as 0.3 µ in diameter or less, may be enclosed in the same fold of Schwann-cell membrane. Outside of the Schwann-cell layer and surrounding the nerve-trunk is a thin layer of connective tissue, which does not send trabeculae into the interior of the nerve. Tracheae and tracheoles accompany the nerve but are not included within the sheaths surrounding a nerve-trunk, even near the termination of the nerve-fibres in muscle. The structure of the cockroach peripheral nerve is compared with that described by previous investigators, with that of other insects, and with invertebrate and vertebrate nerve.

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