THE NERVOUS SYSTEM 



1335 



tube is arranged in three layers or zones — namely, (i) inner or ependymal, 

 (2) intermediate or mantle zone, and (3) outer or marginal zone. 



The ependymal zone consists of a single layer of elongated cells, connected 

 with the internal limiting membrane. Their bases are directed towards the 

 lumen of the neural tube, and from their apices delicate radial fibres pass 

 outwards to the external Umiting membrane. Amongst these there are 

 some conspicuous cells, called germinal cells. These he close to the ■wall of 

 the neural tube, and by their proliferation they give rise to (i) ependymal 

 cells, (2) spongioblasts, and (3) neuroblasts. The latter two migrate outwards 

 into the mantle zone. 



The mantle zone consists of branching cellular strands of myelospongium, 

 which radiate from the centre to the periphery. It contains spongioblasts 

 and neuroblasts, and it forms the rudiment of the grey matter of the spinal 

 cord. 



The marginal zone consists of reticula of myelospongium, traversed by 

 nerve-tracts. It is the rudiment of the white matter of the spinal cord. 



The cells of the Wcdl of the neural tube as they undergo differentiation 

 form two groups — spongioblasts, which form neuroglia, and neuroblasts or 

 ganglionic cells. 



Spongioblast 



GenninalCell 



Fig. 



Young Neuroblast 



. Axon of Neuroblast 



Spongioblast I 



Neuroblast 



358. — The Celxs of the Spinal Cord of the Human Embryo 

 (L. Testut's • Anatomie Humaine ') (after His). 



Development of Neuroglia. — ^This tissue consists of (i) ependymal cells, 

 (2) neuroglia -cells, and (3) neurogha-fibres, all of which are derived from the 

 spongioblasts. These spongioblasts traverse the entire thickness of the wall 

 of the neural tube, and are disposed in strands which radiate outwards from 

 the lumen of the tube, giving off lateral processes. The spongioblasts adjacent 

 to the lumen of the tube form the ependymal cells, and these form the internal 

 limiting membrane. The remaining spongioblasts form the neoroglla'-cells. 



NeurogUa-cells have many branches, and are spoken of as glia-cells or 

 spider-cells. The neuroglia-Qbres are fibrillations of the peripheral protoplasm 

 of the cells, from which they become differentiated. 



The neuroblasts lie in groups within the mantle layer, and they give rise 

 to the nerve-cells of the spinal cord. Each cell is primarily unipolar and 

 pear-shaped. It has a prominent nucleus, and the body is prolonged into a 

 process or pole, whch represents the axon or axis cylinder process of a nerve- 

 fibre. Subsequently the pear-shape is lost, due to the formation of secondary 

 processes or dendrites, the cell being now multipolar. 



The lateral walls of the neural tube become thickened owing to cell-pro- 

 liferation, but ventrally and dorsally it remains thin, these parts forming the 



