DEVELOPMENT OF NERVOUS SYSTEM 



755 



Before the caudal extremity of the tube is entirely closed, its oral end undergoes marked 

 enlargement and becomes distended into three vesicular dilations, the anterior, middle, and 

 posterior primary brain vesicles. The anterior of these primary vesicles give off a series of 

 secondary vesicles and by these, followed by further dilations, flexures of its axis, and by 

 means of locahzed thickenings of its walls, the portion of the tube included in the three primary 

 vesicles develops into the encephalon or brain of the adult. The remainder of the tube becomes 

 the spinal cord. This latter portion retains the simpler form. By the proliferation and migra- 

 tion laterally of the cells lining this portion of the tube, there results a comparatively even 

 bilateral thickening of its walls so that the mature spinal cord retains a cylindrical form through- 

 out its length. 



The proliferating and migrating cells of the wall of the neural tube are known as germijial 

 cells. The products of their division are apparently indifferent at first, but later they become 

 differentiated into two varieties: (1) spongioblasts, or those cells which will develop into neu- 

 roglia, and (2) neuroblasts, or those which will increase in size, develop processes and become 

 nerve cell-bodies. As described below, the processes given off by a neuroblast are of two general 

 characters: (1) a long process or axone which goes to form nerves, nerve roots, and nerve fasciculi, 

 and (2) dendritic processes which are numerous, branch much more frequently and extend but 

 a short distance from the cell-body. An adult ceU-body with all its processes is known as a 

 neurone and the neuroblasts of the developing system become transformed into the neurones 



Fig. 596. — Di.\grams of Transverse Sections of Embryonic Spinal Cords showing the 

 Migration of the Cells of the Ganglion Crest to form the Spinal and Sympathetic 

 Ganglia and the Origin of the Dorsal and Ventral Roots of the Spinal Nerves. 



A, a stage following D^ of fig. 595. B, a later stage in which the ganglia and the components of 

 the nerve are assuming their form resulting from the further migration and from processes 

 being given off by the neuroblasts. 



Ectoderm 



— Spinal ganglion 



Neural 



Sympathet 



of the varying sizes, shapes, and arrangements of processes characteristic of different divisions 

 and localities of the nervous system. Usually the first process to be noted is that which will 

 become the axone or nerve fibre. 



Neurones whose cell-bodies belong to the peripheral nervous system are not developed 

 within the walls of the neural tube or central nervous system at all. These, comprising the 

 spinal ganglion neurones and those of the sympathetic system, are derived from the cells of the 

 ganglion crest. The wedge-shaped lamina of cells, comprising the ganglion crest, through 

 rapid cell division, gradually extends outward and ventralward over the surface of the neural 

 tube along either side. Soon the proliferation becomes most active in regions corresponding to 

 the mesodermic somites or primitive body segments and this, together with the stress of the 

 growing length of the body, results in the ganglion crest (originally a lamina) becoming seg- 

 mented also. The segments or localised cell masses thus formed are the beginning not only of 

 the spinal gangha, but also of the gangha of the entire sympathetic system. The cells of the 

 crest niigrate to assume a more lateral position, and then occurs a separation of their ranks. 

 A portion of them remain in a dorsolateral position near the wall of the neural tube and develop 

 into the neurones of the spinal ganglia (the sensory neurones of the spinal nerves), but others 

 wander further out into the periphery and become the neurones of the sympathetic. Certain 

 of those of this more nomadic group of cells settle within the vicinity of the vertebral column and 

 by sending out their processes, form the gangliated cord or the proximal chain of sympathetic 

 ganglia; others migrate further, but in more broken rank, and become the ganglia of the pre- 

 vertebral plexuses (as the cardiac, cocliac and hypogastric plexuses), or the scattered intermediate 

 chain of ganglia; while stiU others wander into the very walls of the peripheral organs and 



