CHAPTER II. 



Fundamental Conceptions : Ganglion-cell and Xeevb. 



The significance and position of the central neryous system of the 

 Vertebrates can only be understood when one takes into consideration its 

 development, its relation to the peripheral nerve-endings, and to the organs 

 of special sense. 



The central apparatus stands in no wise so isolated or so separated, 

 throxigh morphological or physiological differences, from the peripheral 

 apparatus as it was, until recently, supposed to be. 



Among both vertebrates and invertebrates both systems are derived 

 from the outer embryonic layer: from the epiblast. In vertebrates a part of 

 this thin lamella forms a deep, longitudinal groove which, gradually closing 

 in and separating from the remainder of the epiblast, becomes the tubular 

 fundament of the central nervous system. Another part of the epiblast, 

 lying close beside the groove on either side, forms the fundament of the 

 spinal and cranial ganglia. Many widely-disseminated places produce cells 

 which, even in the higher animals, either remain in the periphery and form 

 cutaneous sense-organs or they sink more or • less deeply and form the 

 fundament of other sense-organs; for example, the olfactory or auditory 

 apparatus or the apparatus of equilibration. This relatively simple picture 

 becomes somewhat more complicated in that many fundaments, which 

 among invertebrates remain completely peripheral, among the vertebrates 

 lie close beside the central system, fusing with it. It is further complicated 

 in that, when the neural groove is once closed, cell-groups wander out from 

 it into the periphery, there later to become independent and scattered 

 ganglia. 



The longitudinal, laminated, epithelial plate which curved in to form 

 the groove representing the fundament of the central nervous system is 

 called the medullary plate. Very early there appear in it, in all classes of 

 vertebrates, changes which lead to the formation of different kinds of cells. 

 Among the epithelial cells, and formed from them, appear the Germ-cells: 

 large, round, protoplasmic structures, — the fundaments of the future 

 ganglion-cells. The axis-cylinders grow from them later, and still later 

 numerous other processes arise from the cell-body, thus stamping the cell 

 as a multipolar one. 



Epithelial cells remain, in part, as the boundary of the central canal of 



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