ELEMENTS OF THE CENTEAL NERVOUS SYSTEM. 503 



j placed in the ependymal layer between its radially arranged cells as they pass 

 1 in towards the internal limiting membrane ; and the protoplasm of the germinal 

 cells forms part of the syncytium. 



At one time it was imagined that the germinal cells were embryonic nerve-cells, 

 [the parent-cells of the real neuroblasts, and that the whole of the rest of the 

 i syncytium represented the supporting tissues, which in the adult form the neuroglia. 

 But "it is now known that from the proliferation of the germinal cells, in which 

 c mitotic figures can usually be seen, some cells are formed which become ependymal 

 epithelium, and others which migrate peripherally into the mantle layer. There, 

 ?'' while forming part of the mantle syncytium, they undergo further proliferation 

 i! and some of the resulting cells develop into spongioblasts, which constitute the 

 i. supporting framework, the embryonic neuroglia; others become rudimentary nerve- 

 cells or neuroblasts, and others again are known as indifferent cells. The latter 

 are destined to undergo further subdivision, and become the parents of more 

 spongioblasts and neuroblasts. 



From this it is clear that the greater part all except the germinal cells 

 of the syncytium, which is known as the myelospongium, is not merely supporting 

 neuroglial tissue, as was once supposed, but is the rudiment of both neuroglia and 

 true nervous tissues. 



The details of the process by which the neuroblasts become dissociated from 

 the neuroglial network are quite unknown. It is commonly supposed that a 

 spherical cell in the mantle layer that is to be transformed into a neuroblast 

 frees itself from the syncytium, and remains for a time independent and wholly 

 unattached amidst the meshes of the neuroglial network : it is supposed further 

 that its true nature as a neuroblast becomes revealed when it takes on a pear- 

 shape, and a protoplasmic process, the stalk of the pear, pushes its way into some 

 other part of the nervous system, or out of it into the mesoderm to reach some 

 muscular or glandular tissue, and becomes the axis cylinder process or axon of 

 the nerve-cell. 



Such an interpretation of the appearances exhibited in the walls of the neural 

 tube at the end of the first month is adduced in support of a view concerning the 

 constitution of the nervous system known as the neurone theory. "Neurone" is the 

 term applied to a nerve-cell and all its processes ; and the neurone doctrine assumes 

 that there is no continuity whatever between the substance of one neurone and that 

 of another, such as occurs in Hydra (Fig. 439), and that the functional connexions 

 between them are brought about merely by the contact of the processes of one 

 element with the processes, or the cell-body itself, of another element. In accord- 

 ance with this conception the facts of embryology are supposed (by His) to demon- 

 strate that when the axon grows out from a previously spherical and unattached cell 

 it is able to push into the surrounding tissues, and, as it were guided by some instinct, 

 eventually finds its way to that particular area of skin, muscle, gland, or other part 

 of the body where nature intends it to go. 



This is the current teaching in regard to the neurone-theory ; and it is supposed 

 to have been conclusively demonstrated by the facts revealed not only by embryo- 

 , logy and the study of the minute structure of the nervous system, but also by the 

 . phenomena of degeneration and regeneration. Harrison has shown that the out- 

 growth of processes can be witnessed in the living nerve-cells of the frog. There 

 ; are certain facts, however, which have always led some anatomists to refuse to 

 believe in the validity of the neurone doctrine as a true expression of the real 

 constitution of the nervous system. It has been clearly demonstrated by Graham 

 Kerr that at a very early stage of development the neural syncytium of the spinal 

 medulla (of the mud-fish Lepidosiren) is in free and uninterrupted continuity with 

 the protoplasm of the muscle-plart>e, which lies in contact with the neural tube ; and 

 no stage is known in which these connexions do not exist. When, in the course of 

 the subsequent growth of the embryo, the muscle-plate becomes removed further 

 , and further away from the central nervous system the protoplasmic strand, which 

 links them the one to the other, gradually becomes stretched and elongated. As 

 the neuroblast matures its chemical constitution becomes modified; it becomes 

 specialised in structure to fit it for the peculiar functions it has to perform. These 



