930 
ME. J. LOCKHAET CLAEKE ON THE DEVELOPIMENT OF 
free surfaces of the walls have a shaggy structure consisting of tine fibres, by which they 
are connected with the walls of their neighbours, and apparently with the nucleated 
investment of the nerve-fibres which run between them *. Sometimes, in consequence 
probably of the action of the chromic acid, the cell-wall is removed to a little distance 
from the surface of the contained cell, but is still connected with it by fine fibres or 
processes. Now in the case of the large nerve-cells of the spinal cord, like those repre- 
sented in fig, 12, cc the thick cell- walls, as development advances, form part of the 
surrounding reticular tissue, with which the surface of the contained cells, still retaining 
a thin investment, is in a similar way connected by processes. Sometimes there remains 
around the stellate cell a more or less circular space enclosed by what seems to be the 
inner surface of the original cell-wall, of which the outer portion has blended Avith the 
surrounding tissue. 
In the account thus given of the development of the spinal cord, I have carefully 
refrained from indulging in any theoretical views, and have confined myself solely to a 
faithful description of what was actually seen. A few remarks, however, are required 
in further explanation of some of the observed facts, and in reference to the conclusions 
that may be drawn from them. 
We have seen that in its earliest stage of development, the spinal cord consists of a 
canal surrounded only by one uniform or homogeneous layer of small cells or nuclei, 
which are not distinguishable from each other in appearance, and are so closely aggre- 
gated as to seem in actual contact. To call this single layer the epithelium of the cord, 
appears to me about as incorrect as it would be to call the germinal membrane of the 
o\aim the mucous or internal of the two layers into which it immediately separates ; for 
in the second stage of its development, we find that this single and homogeneous layer 
constituting the entire substance of the cord, while it continues to increase in depth, 
* The connexion of nerve-cells with each other by means of prolongations of their sheaths is well seen 
in many of the Invertebrata. Fig. 33 shows such a connexion between five cells, from one of the groups of 
ganglia composing the sub oesophageal mass of the common Slug. The central space between these cells is 
occupied by portions of nerve-fibres running in different directions, as well as by connective tissue con- 
tinuous with prolongations of the cell-sheaths. It is not, however, denied that some of the processes contain 
prolongations from the interior of the cells. These cells differ considerably both in size and shape. Many 
of them are enormous. In fig. 33, Plate XL VIII. they are magnified only 420 diameters, and in some parts 
of the ganglion they reach the prodigious size represented in fig. 34 under the same magnifying-power. The 
nuclei within the cells are also of extraordinary dimensions, and fiUed with exceedingly coarse granules, 
amongst which are a variable number of round nucleoli enclosing some finer granules. Between the larger 
cells are others of a much smaller but variable size, many of them not exceeding one-fourth the diameter of 
the nuclei of those represented in fig. 33. A great number of the cells are pyriform, and lie side by side in 
regular rows. The cephalic or supraoesophageal ganglion has an entirely different structure. It consists 
of two lobes, united above the oesophagus by a thick transverse band or commissure. The portion of each 
lobe on the side of the median line is a globular and finely granular mass, traversed by a multitude of 
exceedingly fine fibres proceeding from a somewhat hemispherical layer of small and closely aggregated 
cells, which partially encloses the inner mass, and forms the lateral crust of the ganglion. 
