THE NERVOUS SYSTEM. 451 



nervous system connected by means of a series of segmental nerves with per- 

 ipheral structures (receptors and effectors) and exhibiting at its anterior ex- 

 tremity a higher development .and specialization in both its peripheral and 

 central parts. 



The general features of the typical vertebrate nervous system are best 

 revealed by a brief examination of certain stages in its development. 



The entire nervous system, except the olfactory epithelium and parts of 

 certain ganglia (see p. 452), is derived ontogenetically from an elonga f ed plate 

 of thickened ectoderm, the neural plate. This plate extends longitudinally in 

 the axis of the developing embryo, its position being usually first indicated 

 externally by a median groove, the neural groove (Fig. 410), the edges of the 

 plate being elevated into the neural folds (Fig. 411). The neural folds are 

 continuous around the cephalic end of the plate, but diverge at the caudal 

 end, enclosing between them in this region the blastopore. Even at this stage, 

 the neural plate is usually broader at its cephalic end, thereby indicating already 

 the future differentiation into brain and spinal cord (Fig. 413). The neural 

 folds now become more and more elevated (Fig. 412), presumably due in 

 part to the growth of the whole neural plate, and finally meet dorsally and fuse, 

 thus forming the neural tube (Figs. 72 and 429). The fusion of the lips of the 

 neural plate to form the neural tube usually begins somewhere in the middle 

 region of the plate and thence proceeds both forward and backward (Fig. 119). 

 The last point to close anteriorly is usually considered as marking the cephalic 

 extremity of the neural tube, and is called the anterior neuropore. 



Even before the neural plate closes to form the tube, there is often a differen- 

 tiation of cells along each edge, forming an intermediate zone between the 

 neural plate and the non-neural ectoderm (Fig. 429). As the neural plate 

 becomes folded dorsally into the neural tube these two zones are naturally 

 brought together at the point of fusion of the dorsal lips of the neural plate. 

 The two zones thus brought together are not included in the wall of the neural 

 tube, but form a paired or unpaired ridge of cells lying along its dorsal surface. 

 This ridge of cells is called the neural crest (Fig. 429). Later, each half of the 

 neural crest separates from the other half and from the neural tube and passes 

 ventrally down along the sides of the tube, at the same time becoming trans- 

 versely divided into blocks of cells (Fig. 434). These masses of cells are the 

 rudiments of the cerebrospinal ganglia and differentiate into the afferent per- 

 ipheral neurones, and into some at least of the efferent peripheral visceral neu- 

 rones (sympathetic) as well as some other accessory structures (see pp. 489 

 to 494). The peripheral processes of these ganglion cells (afferent peripheral 

 nerve fibers) pass to the receptors, the central processes (afferent root fibers) enter 

 the dorsal part of the nerve tube (Fig. 430). In the case of the special sense 

 organs there is an interesting tendency on the part of portions of the neural 



