To all intents and purposes the same thing is seen in the termination 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, therefore, 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 transversely 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-oesophageal 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. {52}This is the great characteristic of the advancement of the central nervous system among the Invertebrata, its concentration in the region of the head. It may be called the principle of cephalization, and is characteristic not only of higher organization in a group, but also of the adult as distinguished from the larval form. Thus in the imago greater concentration is found than in the caterpillar.