CENTRIFUGAL FIBRES IN THE AVIAN VISUAL SYSTEM

A study has been made of the origin and course of the centrifugal fibres in the visual pathway of the pigeon using the Nauta method. Lesions in the mid-brain involving the isthmo-optic nucleus result in fibre degeneration which can be traced through the isthmo-optic tract to the chiasma and thence into the contralateral optic nerve and retina. In the retin a severe degeneration is found throughout the optic nerve layer, and occasional degenerating fibres can be traced through the ganglion cell layer to the inner aspect of the bipolar cell layer. Here they terminate in endings similar to those described by Cajal (1889) and Dogiel (1895) in relation to amacrine cells. The projection to the retina is completely crossed. Counts of the number of cells in the isthmo-optic nucleus indicate that the number of centrifugal fibres is approximately 10000; they form 1 % of the total number of fibres in the optic nerve. The isthmo-optic nucleus receives afferents from the tectum, and in this projection there would appear to be a well-defined organization.

FINE STRUCTURE OF THE EYE OF A CHAETOGNATH

Electron microscopy reveals a star-like pigment cell at the center of the eye of the arrow-worm, Sagitta scrippsae. Between the arms of the pigment cell are clusters of photoreceptor cell processes, each process consisting of: (1) a tubular segment containing longitudinally arranged microtubules about 500 A in diameter and 20 µ in length; (2) a remarkable conical body, composed of cords and large granules, situated at the base of the tubular segment; and (3) a connecting piece which, like that of rods and cones, connects the process with the sensory cell proper and through which runs a fibrillar apparatus consisting of nine peripheral double tubules. Beneath the connecting piece lies a typical centriole with a striated rootlet. The receptor cell process is deeply recessed into the sensory cell which may possess a corona of microvilli at its inner surface. A nerve fiber arises from the outer end of the cell and passes into the optic nerve. Additional features are some supporting cells, an external layer of flattened epithelial cells, and an over-all investment of basement membrane and thick fibrous capsule. The fine structure and function of these elements of the eye are discussed in relation to earlier studies with the light microscope. The ciliary nature of the photoreceptor cell process in S. scrippsae points to a probable evolutionary relationship of chaetognaths to echinoderms and chordates.

FURTHER OBSERVATIONS ON THE STRUCTURE OF MYELIN SHEATHS IN THE CENTRAL NERVOUS SYSTEM

Direct evidence has been presented to confirm the existence of a spiral in the myelin sheaths of the central nervous system. An account of some of the variations in structure of central myelin sheaths has been given and it has been shown that the radial component of myelin sheaths has the form of a series of rod-like thickenings of the intraperiod line. These thickenings extend along the intraperiod line in a direction parallel to the length of the axon. The relative position of the internal mesaxon and external tongue of cytoplasm has been determined in a number of transverse sections of sheaths from the optic nerves of adult mice, adult rats, and young rats. In about 75 per cent of the mature sheaths examined, these two structures were found within the same quadrant of the sheath, so that the cytoplasm of the external tongue process tends to lie directly outside that associated with the internal mesaxon. The frequency with which the internal mesaxon and external tongue lie within the same quadrant of the sheath increases both with the age of the animal and with the number of lamellae present within a sheath. The possible significance of these findings is discussed.