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-cesophageal 
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 eranial 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, 
