THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 51 



the nervous material surrounding it ; all the nerves supplying the 

 new tail arise from the uninjured spinal cord above, the central 

 canal with its lining layer of epithelial cells alone grows into the 

 new-formed appendage. 



To all intents and purposes the same thing is seen in the termi- 

 nation of the spinal cord in a bird-embryo; more and more, as the 

 end of the tail is approached, does the nervous matter of the spinal 

 cord grow less and less, until at last a naked central canal with 

 its lining epithelium is alone left to represent the so-called nerve- 

 tube. 



All these different methods of investigation lead irresistibly to 

 the one conclusion that the tubular nature of the central nervous 

 system has been caused by the central nervous system enclosing to a 

 greater or less extent a pre-existing, non-nervous, epithelial tube. 



This must always be borne strictly in mind. The problem, there- 

 fore, which presents itself is the comparison of these two factors 

 separately, in order to find out the relationship of the vertebrate to 

 the invertebrate. The nervous system without the tube must be 

 compared to other nervous systems, and the tube must be considered 

 apart from the nervous system. 



The Principle of Concentration and Cefhalizatiox. 



The central nervous system of the vertebrate resembles that of 

 all the Appendiculata in the fact that it is composed of segments 

 joined together which give origin to segmental nerves. There is, 

 however, a great difference between the two systems : the division 

 into separate segments is not obvious to the eye in the vertebrate 

 nervous system, while in the invertebrate we can see that it is 

 composed of a series of separate pairs of ganglia joined together 

 longitudinally by nervous strands known as connectives and trans- 

 versely by the nerve-commissures. Such a simple segmented system 

 is found in the segmented worms, and in the lower arthropods, such 

 as Branchipus, no great advance has been made on that of the annelid. 

 In the higher forms, however, a greater and greater tendency to fusion 

 of separate ganglia exists, especially in the head-region, so that the 

 infra- (esophageal ganglia, which, in the lower forms are as separate 

 as those of the ventral chain, in the higher forms are fused together 

 to form a single nervous mass. 



