918 
ME. J. LOCKHAET CLAEKE ON THE DEVELOPMENT OE 
and has gradually blended with the anterior grey substance ; while its inner border 
forms the epithelial layer (s) surrounding the central canal. Along the sides of the 
canal, the epithelium, as regards its constituent elements, differs but little in appear- 
ance from the rest of the grey substance. The only apparent differences are, that its 
nuclei are less closely aggregated ; that the fibres radiating through their interspaces 
from the verge of the canal, and which are thicker, are consequently seen to be more 
continuous and branched ; and that a few of its nuclei are larger, while others are more 
elongated or fusiform at right angles to the axis of the canal. Nor is it bounded 
externally by any definite line, but is continuous, as a nucleated network, with that of 
the anterior cornu. From the posterior margin of the elliptical canal its radiating fibres 
extend outward and backward into the posterior cornua, and directly backward between 
them into the posterior median fissure. In front of the canal, however, the epithe- 
lium has a somewhat different arrangement. At this part it forms a deeper and more 
distinct layer, containing more of the fusiform nuclei, which are elongated in a direc- 
tion forward, and terminate at each extremity in a fibre. At the sides of this layer the 
nuclei with their fibres are curved inward, but become progressively straighter as they 
approach the middle line (see figs. 4 & 8). Posteriorly their processes or fibres are 
thicker, and attached by their extremities to the margin of the canal ; while anteriorly 
they converge across the anterior commissure, as a conical network, into the anterior 
median fissure, where they become directly continuous with the fibres of the pia mater 
and connective tissue enveloping numerous blood-vessels and derived from the chcum- 
ference of the cord {m, figs. 4 & 8). In many cases the epithelium mfrooit of the canal 
differs but little in appearance from the rest of the layer, except that the principal fibres 
of the network converge forward to join the process of pia mater in the anterior median 
fissure. In such cases it has a general resemblance to that seen in fig. 9, Plate XLV., 
which exactly represents a portion of the canal and surrounding grey substance of a 
human foetus of nine weeks, magnified 420 diameters. It was taken from the same 
foetus as the section represented in fig. 8, Plate XLVI. o is the anterior part of the canal ; 
s, s, between the canal and the outer line, is the layer of epithelium, which on each side, 
however, is seen to be gradually continuous with the anterior grey substance, f; while 
in front it terminates in a conical network of connective tissue interspersed with nuclei, 
and continuous (at m) with the pia mater of the surface. 
I have next to show that the elements of the grey substance, which in fig. 3 were 
nearly uniformly alike, have in fig. 4 become more or less modified both in structure 
and arrangement. In fig. 3 every nucleus had a plain or smooth appearance, without 
any trace of granular contents or of a distinct enveloping membrane. In fig. 4, however, 
both these modifications of structure had taken place ; and although the observation 
of this fine distinction might seem to be trivial and unimportant, it will nevertheless 
be seen, as we proceed, to be well worthy of attention. It was also observed that 
in section 3 the nuclei were very nearly of the same size in all parts of the grey sub- 
stance. In section 4, however, those of the anterior grey substance had everwhere 
