VII.] THE SPINAL CORD. 185 



The posterior roots of the spinal nerves enter the cord 

 near its posterior surface, and at this point the posterior grey 

 columns are formed in a similar way to the anterior. In 

 some cases also the rudiment of a posterior white column 

 may be seen at the junction of the nerve with the epiblast of 

 the canal. The fibres of the posterior root cannot be traced 

 so far into the cord as those of the anterior root. 



The grey matter of the cord seems undoubtedly to be formed by a meta- 

 morphosis of the external cells of the epiblast of the neural tube, and is 

 directly continuous with the epithelium ; there being no strong line of demarcation 

 between them. Whether the fibres which traverse it, and which seem to be 

 partly nervous and partly connective tissue in their nature, are derived from 

 mesoblast or epiblast our observations have not enabled us to determine. 



The white matter which caps the grey mass, and which forms the com- 

 mencement of the anterior white column, is a peculiar tissue. It consists of a 

 network of fibres somewhat resembling the connective tissue network of the 

 white matter of the adult cord, to which it has a further likeness in not being 

 easily stained by carmine. The fibres of which it is composed have a general 

 tendency to be disposed in radiating septa, a peculiarity which is especially 

 noticeable with low powers. Along the fibres and more especially in the septa, 

 numerous highly refracting granules are embedded, and in the meshes pale 

 spherical nuclei with nucleoli are to be seen. The boundary between the white 

 and grey matter is very sharply defined, and we have always failed to trace the 

 fibres of which we are speaking into the fibres present in the grey matter, though 

 Lockhart Clarke (Phil. Trans. 1862) asserts that they are continuous. Nor can 

 the fibres of the nerve-roots be seen to come into connection with these same 

 fibres. It has generally been assumed that the white matter like the grey is 

 derived from the epiblast : this does not however appear ever to have been 

 clearly proved, while the peculiarities of the tissue, and the fact that it first 

 appears at the origin of the spinal nerves, might seem to indicate that it is 

 directly derived from the mesoblast surrounding the cord ; a view which we are 

 inclined to accept. 



On the fourth day there is no trace of either an anterior 

 or a posterior fissure, and in the lumbar region the shape of 

 the spinal canal is not very different from what it was on the 

 third day. It appears in sections as a narrow slit dilated 

 somewhat at either end (Fig. 47). The epithelium surround- 

 ing the slit is still very thin, especially above and below, but 

 at the anterior end forms a somewhat arched projection 

 with the convex surface turned downwards. 



On the fifth and sixth days important changes take place. 



By the great increase of the grey matter, which now 

 comes to form the chief mass of the cord, the epithelium is 

 reduced to a thin layer of cells immediately surrounding the 

 canal. 



In the dorsal region, the side walls of the laterally com- 



