THE SPINAL COED IN MAN, MAMMALIA, AND BIEDS. 
931 
undergoes a differentiation into two distinct layers — an inner, constituting the true 
epithelium, and an outer, constituting the grey substance ; and although the former 
does not undergo the histological and morphological changes which subsequently take 
place in the mucous layer of the germinal membrane, yet it is probable that after the 
production of the outer or grey substance, it differs in its histological character from the 
originally homogeneous layer. This differentiation of structure proceeds very gradually, 
and is not at first marked by any decided line of separation, or by any difference in the 
appearance of the structural elements. At the same time there is gradually formed 
around, and apparently secreted from, the small cells or nuclei, a granular substance 
that forms into processes or fibres, and constitutes a continuous network by which all 
the nuclei or cells of both layers are uninterruptedly connected. In the grey substance 
itself there is at first little or no apparent difference in structure between its anterior 
and posterior portions, although in each portion darker and more-closely aggregated 
groups of nuclei may be observed in connexion with the roots of the nerves. But as 
development progresses, a diversity of structure ensues ; for while the nuclei of ihe^poste- 
rior grey substance, although rather more granular than at first, have scarcely advanced 
in size, those of the anterior substance have increased to double their original diameter, 
and are connected by thicker fibres, which form a coarser and more granular network. 
At the same time, around the separate groups of the latter substance, the granular net- 
work (seen in fig. 35, Plate XLVIII.) between the nuclei assumes a more sponge-like 
structure, as represented at fig. 12. Meanwhile, within the group, there are formed 
from the nuclei a number of large, roundish or irregular but adjacent cells, with thick 
nucleated walls (x w, fig. 12). It is quite evident that the nucleated tissue constituting 
the walls of these cavities and the network around them is in every respect similar in 
appearance to that which is very commonly assumed by the connective tissue of parts 
external to the cord, as may be seen, for instance, at b, fig. 23, Plate XLVII., which repre- 
sents a portion of the connective tissue on the outer surface of one of the intervertebral 
ganglia, with the substance of which, however, it is directly continuous. 
It appears, then, that in these early stages of development there are at least two kinds 
of free nuclei in the grey substance of the cord. The one kind appear to develope the 
general network of tissue which pervades the entire structure, but proceed no further ; 
whereas each of the other kind, while connected with this network as well as with 
nerve-fibres, developes a nucleated cell with a nucleated wall which is still connected, 
and ultimately blended, with the surrounding reticular structure. In the cells of the 
intervertebral ganglia, although the process appears to be essentially the same, there is 
some difference in the appearance they present in the earlier stages of development. 
Fig. 23, «, Plate XLVII. is a small portion of one of the intervertebral ganglia of a 
foetal sheep, 1 inch long. Fig. 35, Plate XLVIII. is a portion of the dark group of 
nuclei in the anterior grey substance of the cord, destined to be developed into large 
nerve-cells, from the same foetus. In the former the nuclei are joined by more sharply 
defined fibres, and there is an absence of the delicate granular network surrounding 
