THE NERVOUS SYSTEM OF VERTEBRATES 



335 



except their nuclei, so that the neuroglia appears as a close felt-work 



of fibres, to which here and there nuclei are attached. These cells are 



formed from the most superficial layer of the invaginated epiblast, 



and are spoken of as spongioblasts. The deeper layer of cells, which 



are to give rise to the permanent nerve-cells, and are therefore known 



as neuroblasts, rapidly divide and form a thick layer surrounding the 



internal layer of spongioblasts, through which pass the peripheral 



processes of the latter. When first formed these cells have no processes. 



Later on each neuroblast acquires a pear shape, the stalk of the pear 



having a somewhat bulbous ex- 



tremity (Fig. 142). The stalk con- 



tinually elongates, and the elongated 



process may leave the spinal cord 



altogether and grow outwards to 



any part of the body of the embryo, 



or may pass to other parts of the 



central nervous system. This long 



process of the developing nerve- 



cell is known as the axon. Some 



time after its formation other pro- 



cesses grow out from the cell, which 



soon branch and end in the im- 



mediate neighbourhood of the cell. 



The axons of the cells near the 



ventral part of the neural tube 



grow out to the different muscles of FIG. 142. Neuroblasts from the spinal 



the body, where they end in close c r . d o a chic ^ T b T- 



. J .A, three neuroblasts stained to show 



connection With the muscular fibres neuro-fibrils ; a, a bi-polar cell. 



by an arborisation which forms the B ' me nta[ cone 3 * c showing the ' incre " 

 end-plate. They provide an efferent 



path for impulses running from the central nervous system to the 

 musculature of the body. The afferent channel is formed in a some- 

 what different manner. Even before the neural groove has closed in, 

 a thickening of the epiblast is seen immediately external to the 

 groove on each side. This thickening becomes divided into a series 

 of collections of cells lying immediately under the epiblast on the 

 lateral and dorsal surface of the neural canal. The cells, which are 

 at first round or oval, send off two processes in opposite directions so 

 that they become bi-polar (Fig. 143). One process passes into the 

 central nervous system, where it divides, some of its branches being 

 distributed in the nervous system at the same level, while others 

 run a considerable distance towards the head immediately outside 

 the tube of nerve-cells. The other process grows downwards, 

 along with the processes from the ventral cells of the tube, 



