THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 47 



the whole dorsal area of the cord is taken up by a peculiar non- 

 nervous wedge-shaped mass of tissue. At its first formation this 

 portion of the cord is formed exactly in the same manner as the rest 

 of the cord ; instead, however, of the nervous material invading the 

 dorsal part of the tube to form the posterior fissure, it has been from 

 some cause unable to do so, the walls of the original non-nervous 

 tube have become thickened dorsally, been transformed into this 

 peculiar tissue, and so caused the peculiar appearance of the cord 

 here. The nervous parts have not suffered in their development ; 

 the mechanism for walking in the bird is as well developed as in 

 any other animal ; their position only is different, for they still retain 

 the original ventro-lateral position, but the non-nervous tube, the 

 remains of the old intestine, has undergone a peculiar gelatinous 

 degeneration just where it has remained free from invasion by the 

 nervous tissue. 



Throughout the whole of that part of the nervous system which 

 gives origin to the cranial and spinal segmental nerves, the evidence 

 is absolutely uniform that the nervous material was originally 

 arranged bilaterally and ventrally on each side of the central tube, 

 exactly in the same way as the nerve-masses of the infra-oesophageal 

 and ventral chain of ganglia are arranged with respect to the cephalic 

 stomach and straight intestine of the arthropod. But, in addition, we 

 find in the vertebrate nervous masses, the cerebral hemispheres, the 

 corpora quadrigemina and the cerebellum situated on the dorsal side 

 of the central tube in the brain-region ; this nervous material is, 

 however, of a different character to that which gives origin to the 

 spinal and cranial segmental nerves. How is the presence of these 

 dorsal masses to be explained on the supposition that the dilated 

 anterior part of the nerve-tube was originally the cephalic stomach 

 of the arthropod ancestor ? The cerebral hemispheres are simple 

 enough, for they represent the supra-cesophageal ganglia, which of 

 necessity, as they increased in size, would grow round the anterior 

 end of the cephalic stomach and become more and more dorsal in 

 position. 



The difficulty lies rather in the position of the cerebellum and 

 corpora quadrigemina, and the solution is as simple as it is 

 conclusive. 



Let us again turn to embryology and see what help it gives. In 

 all vertebrates the dilated anterior portion of the nerve-tube does not, 



