THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 55 



together in the caudal region to form what is known as the caudal 

 brain. 



The brain in these animals may be spoken of as composed of 

 three parts — (1) the fused supra-cesophageal ganglia, (2) the fused 

 prosomatic ganglia, and (3) the fused mesosomatic ganglia. Such a 

 brain is strictly homologous with the vertebrate brain, which also is 

 built up of three parts — (1) the part in front of the notochord, the 

 prechordal or supra-infundibular brain, which consists of the cerebral 

 hemispheres, together with the basal and optic ganglia and corre- 

 sponds, therefore, to the supra-cesophageal mass, with its olfactory 

 and optic divisions lying in front of the oesophagus ; (2 and 3) the 

 epichordal brain, composed of (2) a trigeminal and (3) a vagus divi- 

 sion, of which the first corresponds strictly to the fused prosomatic 

 ganglia, and the second to the fused mesosomatic ganglia. Further, 

 just as in the embryo of an arthropod it is possible, with more or 

 less accuracy, to see the number of neuromeres or original ganglia 

 which have fused to form the supra- and infra- oesophageal portions 

 of its brain, so also in the embryo of a vertebrate we are able at 

 an early stage to gain an indication, more or less accurate, of the 

 number of neuromeres which have built up the vertebrate brain. 

 The further consideration of these neuromeres, and the evidence they 

 afford as to the number of the prosomatic and mesosomatic ganglia 

 which have formed the epichordal part of the vertebrate brain, must 

 be left to the chapter on the segmentation of the cranial nerves. 



The further continuation of this process of concentration of 

 separate segments, together with the fusion of the nervous system 

 with the tube of the alimentary canal, leads in the simplest manner 

 to the formation of the spinal cord of the vertebrate from the meta- 

 somatic ganglia of the ventral chain of the arthropod. 



The Antagonism between Cephalization and Alimentation. 



This concentration of the nervous system in the head- region, 

 together with an actual increase in the bulk of the cephalic nervous 

 masses, constitutes the great principle upon which the law of upward 

 progress or evolution in the animal kingdom is based, and it illus- 

 trates in a striking manner the blind way in which natural selection 

 works; for, as already explained, the central nervous system arose as 

 a ring round the mouth, in consequence of which, with the progressive 



