912 MR. J. LOCKHART CLARKE ON THE DEVELOPilENT OF 
In a foetal sheep nine lines in length, a transverse section was made through the 
spinal column and cord, at the upper part of the lumbar enlargement. The section of 
the cord was of an ovoid form, with its longer axis before-backward, but was somewhat 
broader at its anterior than its posterior half (see Plate XLV. fig. 1). The grey substance 
occupied nearly the whole of its area, but formed an irregular outline which was not 
concentric with the surface of the section. The central canal was a mere slit or fissure 
extending backward and forward to near the surface of the grey substance, particularly 
at its posterior end. Around this fissure was a somewhat dark layer, which was broad 
along the sides, as well as in front, where it formed a nearly semicircular projection 
beyond the rest of the grey substance, and was covered externally by the rudiment of 
the anterior commissure (fig. 1, a). Posteriorly it reached quite to the surface of the 
cord, being hitherto uncovered by the white columns, and was joined to its fellow of the 
opposite side by a shallow bridge of the same kind of substance. When examined by 
means of high magnifying-powers, this layer was seen to consist of closely aggregated 
nuclei connected together by a continuous network of short fibres. The nuclei in size 
were a little unequal, but their average diameter was about equal to that of the blood- 
globules. In shape, also, they were somewhat diversified from the round to the oval, 
the pyriform or the variously angular ; but were all intermingled without regularity or 
order. From the inner border of the layer, that is, from the verge of the canal, the 
fibres, although they formed an irregular network with the densely aggregated nuclei, 
had nevertheless a tendency to radiate outward to the rest of the grey substance. 
From the outer side of the layer, at its posterior extremity, a somewhat dark and ciuved 
process (d), forming the lateral boundary of the posterior grey substance, extended 
outward and terminated in a roundish but imperfectly defined mass (S'), midway between 
the anterior and posterior extremity of the section. This mass, however, was uninter- 
ruptedly continuous with the outer border of the layer surrounding the canal, as well 
as with the anterior grey substance, by means of a paler portion composed of similar 
material. The surface of the mass and concave surface of the process which jomedit to 
the posterior extremity of the section were overlaid with the rudiments of the posterior 
white column (c, fig. I). In section this column had an oval figure, and covered oiily 
about the two anterior thirds of the posterior grey substance, leaving the other convex 
third entirely uncovered, except by the rudimentary integuments. By means of a sutficient 
magnifying-power, it was clearly seen that at the surface of the grey substance the fine 
network of fibres between the closely aggregated nuclei was directly continuous with the 
fibres of the white column with which it was overlaid. Fig. 2 represents a portion of a 
transverse section of the posterior grey and white substances at their points of junction, 
magnified 420 diameters: a is an inner part of the white column, and 1) is an outer part 
of the grey substance, consisting of nuclei and a Tlose network of fibres continuous 
with a. Through the same network nuclei were connected with some of the fibres of the 
posterior nerve-roots (cZ), which ran obliquely outward and forward, and made then* exit 
at the anterior border of the posterior column, to enter the intervertebral ganglion (e). 
