ii NEKVOUS SYSTEM 83 



Origin of the Central Nervous System. — The first obvious 

 trace of the central nervous system consists of a thickening of the 

 ectoderm of the dorsal surface of the embryo. This thickening 

 extends forwards from the anus, or from a point slightly behind the 

 anus, to the head region, aud is termed the medullary plate. The 

 thickening of the medullary plate is due primarily to the ectoderm 

 cells, or where two layers are present, to the deep-layer cells of the 

 ectoderm taking on a tall columnar form. 



There is also growth in area of the medullary plate and this, 

 in conjunction with the binding down of the medullary plate along 

 the line of the notochord and primitive streak, causes it to become 

 curved from side to side so as to form a gutter or groove — the 

 medullary groove. The, usually conspicuous, lips of this groove 

 are known as the medullary folds. 



As the medullary plate keeps on increasing in width it bulges 

 downwards and laterally into the surrounding mesenchyme and 

 assumes the form of a longitudinally placed tube with a slit along 

 its dorsal wall representing the original opening of the groove. 



Finally the lips of this slit grow towards one another and under- 

 go fusion, so as completely to close in the neural tube, which now 

 separates off from the ectoderm of the outer surface. The closing in 

 of the neural tube commonly commences in the hinder head region 

 and spreads from this point forwards and backwards. 



An interesting modification of this normal mode of origin of the 

 neural tube is found in the case of Lampreys, Zepidosiren and many 

 of the teleostomatous fishes. In this modification of the typical 

 process the increase in bulk of the medullary plate leads to its 

 growing downwards into the underlying tissue as a solid keel. In 

 the middle of this at first solid rudiment a cavity appears secondarily 

 either by the development of a fine intercellular split or by the cells 

 along the axis breaking down. The cavity so formed gradually 

 dilates and eventually there is a neural tube agreeing with that of 

 normal forms. 



The neural tube which has originated in the way described is the 

 rudiment of the central nervous system, its anterior portion becoming 

 relatively enlarged to form the brain while the remainder forms the 

 spinal cord. 



The central nervous system during the period of its development 

 gradually attains to a condition of the greatest complexity and all 

 that will be attempted here is to give an outline sketch of the more 

 conspicuous changes which take place in its general form and in the 

 arrangement of its parts without going into minute detail. 



Spinal Cord. — The spinal cord remains throughout life in the 

 form of a tube the lumen of the tube becoming relatively insignifi- 

 cant while the walls become greatly thickened especially laterally. 

 The relatively small size of the lumen (central canal) is not, as a 

 rule, due merely to its retaining its embryonic dimensions while the 

 walls of the tube are growing in thickness. On the contrary actual 



