explain the relation which they bear to the cells. Remak [1] describes the early condition of the nerves in the following manner: “The substance of the cerebro-spinal nerves of the rabbit, in the third week of embryonal existence, consists of corpuscles, some of which are irregularly spherical, others slightly elongated, having a very delicate filament adhering to them; they are mostly transparent, and arranged in rows without, however, presenting any distinctly perceptible fibrous structure.” And 1. c. page 153, he says, “ A structureless and general globular mass is the original form, from which the primitive fibres of the cerebro-spinal nerves are developed. These primitive fibres are at first varicose, and contain no medulla; most of them pass into the cylindrical form, through the intermediate stage of transitional fibres.”
I have investigated the development of nerve in the foetal pig. The nerves of the foetus have not the shining white colour, presented by those of the adult animal, but are gray and transparent, and the younger the embryo the more striking are these appearances. We are, therefore, quite prepared to find that microscopic investigation shows the white substance of the fibres to be less perfectly or not at all developed. If a nerve, taken from a foetal pig of about six inches in length, be spread out, in the usual mode of preparation by tearing it under water, some fibres are seen which very much resemble those of the adult animal, and which are furnished with outlines almost as dark. The greater part of the substance, however, does not form connected fibres, but consists of separate round globules, or more or less long, irregular little cylinders, arranged with their long axes in the direction of the course of the nerves, having outlines, however, quite as dark as those of the nervous fibres. These appear to be what Remak refers to in the description previously quoted. In addition to them, however, a substance of quite another appearance is seen, which has not the dark outline, does not appear pellucid but granulated, and in which the cell-nuclei are distinctly recognisable.
When the other constituent parts predominate, the nuclei
- ↑ Müller’s Archiv, 1836, p. 148. Respecting the microscopic structure of the brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183.
