932 
ME. J. LOCKHAET CLAEKE ON THE DEVELOPMENT OF 
the nuclei in the latter. As development advances, these nuclei or small cells of the 
intervertebral ganglia simply enlarge, at first, but at the same time are connected 
with each other and with intervening granular nuclei by fine fibres, as shown in 
fig. 24, Plate XLVII. At a later period a very distinct and well-defined nucleus, sur- 
1 ‘ounded by a variously shaped granular mass, makes its appearance within each cell; 
while its surface becomes the cell-wall, which at first is thin, but gradually increases in 
thickness. Now the surfaces of these cells are in connexion not only with the inter- 
vening nucleated fibres (as shown in fig. 24) which are continuous with the connective 
tissue forming the sheath of the ganglion, but also with the walls of the adjacent blood- 
vessels, as seen at c, c in the same figure ; and it is probably through the medium of this 
nucleated tissue that the developing cells are supplied with nutritive fl.uid. Indeed, if 
We except the muscular fibre-cells, with which some of them are provided, the walls of 
the blood-vessels are only a part of the pia mater and connective tissue between them. 
On examining the layer of pia mater which immediately surrounds the cord, it may be 
seen to connect the walls of the blood-vessels which it contains with the sheaths of the 
intervertebral ganglia, and, through this, with the sheaths of their nerve-cells, on one 
side, and on the other side with the connective tissue or pia mater within the cord itself. 
In fig. 36, Plate XLVIII., a a' represent the outer surface of the cord ; b b the outer 
part of one of the intervertebral ganglia ; and c d the intervening layer of pia mater, 
containing blood-vessels, d, e. On its left side, the walls of the vessels {e, d), containing 
some globules, are seen to be connected by a continuous and nucleated network with 
the sheath of the ganglion, b b ; and from the right side of the walls a series of pro- 
cesses or fibres enter the Cord (particularly at a, a!), in which they are continuous with 
the reticular tissue of both its white and grey substance. At d there intervenes, between 
the transversely-cut vessel [d) and the cord, a layer of pia mater, from which similar 
processes are derived. These processes are not branches of the blood-vessels ; and they 
enter the cord around the whole of its circumference. Now, in the anterior median 
fissure, it may be seen beyond all question, as already stated, that this layer of pia 
mater, with the walls of the blood-vessels which it contains, is directly continuous with 
the processes of the epithelium surrounding the front of the canal (see figs. 4, 8, 9, 
& 14) ; and therefore we are warranted in concluding that a similar continuity exists 
mthin and around the remaining parts of the cord. But this is only an dpnon con- 
firmation of what I long ago actually observed and described in the adult cord*. 
And since the sheaths of the nerve-cells have been shown to be continuous with, and 
indeed to form a part of, the reticular connective tissue of the cord, which is itself con- 
tinuous with the pia mater of the surface, it is evident that the processes of the epithe- 
lium, the pia mater and connective tissue within the cord, the walls of the blood- 
vessels, and the sheaths of the nerve-cells must be all uninterruptedly continuous with 
each other. But the sheaths of the nerve-cells are certainly connected with the sur- 
faces of their granular contents ; and in the fully-developed cord, where the ceU- 
* Philosophical Transactions, 1851 and 1859, 
