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The Central Nervous System of the Vertebrata[158].
The formation of the cerebrospinal axis of the Chordata from the medullary plate has already been treated at length (pp. 301-304). Before entering into the consideration of the morphological value of the various parts of this cord, it will be convenient to describe the more important features of its ontogeny. For this purpose the two parts into which the nervous axis becomes at an early period divided, viz. the spinal cord and the brain, may be dealt with separately.
The Spinal Cord, shortly after the closure of the medullary canal, has, in all the true Vertebrata, the form of an oval tube; the walls of which are of a fairly uniform thickness, and are composed of several rows of elongated cells. This cord, as development proceeds, usually becomes vertically prolonged in transverse section, and the central canal which it contains also becomes vertically elongated. The variations in shape of the spinal canal are very great at different periods and in different parts of the body, and an attempt to chronicle them would appear, in the present state of our knowledge, to be quite valueless[159]. [fig. 117], in which the spinal cord of the chick of the third day is shewn in transverse section, illustrates the character of the cord at the stage just described. Up to this time the walls of the spinal canal have exhibited an uniform structure. A series of changes now however takes place, which results in the differentiation (1) of the epithelium of the central canal, (2) of the grey matter of the cord, and (3) of the external coating of white matter.
The relative time at which each of these parts becomes developed is not constant in the different forms.
Fig. 246. Section through the spinal cord of a seven days’ Chick.
pcw. dorsal white column; lcw. lateral white column; acw. ventral white column; c. dorsal tissue filling up the part where the dorsal fissure will be formed; pc. dorsal grey cornu; ac. anterior grey cornu; ep. epithelial cells; agc. anterior commissure; pf. dorsal part of spinal canal; spc. ventral part of spinal canal; af. anterior fissure.
The white matter is apparently the result of a differentiation of the outermost parts of the superficial cells of the cord into longitudinal nerve-fibres, which remain for a long period without a medullary sheath. These fibres appear in transverse sections as small dots. The white matter forms a transparent investment of the grey matter and would seem to contain neither nuclei nor cells[160]. The white matter may from the first form only two masses, one on each side, forming a layer on the ventral and lateral parts of the spinal cord but not extending to the dorsal surface (Elasmobranchii, [fig. 185], W); or it may form four patches, viz. an anterior and a posterior white column on each side, which lie on a level with the origin of the anterior and posterior nerve-roots (the Fowl, Human embryo, etc.). In whichever of these forms the white matter appears, it is always, at first, a layer of extreme tenuity, which rapidly increases in thickness in the subsequent stages, and extends so as gradually to cover the whole cord ([fig. 246]).
The anterior white commissure is formed very shortly after the first appearance of the white matter. The grey matter and the central epithelium are formed by a differentiation of the main mass of the spinal cord. The outer cells lose their epithelial-like arrangement, and, becoming prolonged into fibres, give rise to the grey matter, while the innermost cells retain their primitive arrangement, and constitute the epithelium of the canal. The process of formation of the grey matter would appear to proceed from without inwards, so that some of the cells, which have, on the formation of the grey matter, an epithelial-like arrangement, subsequently become converted into true nerve-cells.