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 CEPHALIZATION. 
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-cesophageal 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. 
