THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 37 



is one, if not the most striking, of the peculiarities which distinguish 

 the vertebrate ; a tubular central nervous system such as that of the 

 vertebrate is totally unlike any other nervous system, and the very 

 fact that the two nervous systems of the vertebrate and arthropod 

 are so similar in their nervous arrangements, makes it still more 

 extraordinary that the nervous system should be grouped round a 

 tube in the one case and not in the other. 



Now, in the arthropod the oesophagus leads directly into the 

 stomach, which is situated in the head-region, and from this a straight 

 intestine passes directly along the length of the body to the anus, 

 where it terminates. The relations of mouth, oesophagus, alimentary 

 canal, and nervous system in these animals are represented in the 

 diagram (Fig. 3). 



Any tube, therefore, such as that of the infundibulum, which 

 would represent the oesophagus of such an animal, must have opened 

 into the mouth on the ventral side, and into the stomach on the 

 dorsal side, and the lining epithelium of such an oesophagus must 

 have been continuous with that of the stomach, and so of the whole 

 intestinal tract. 



Supposing, then, the animal is not turned over, but that the dorsal 

 side still remains dorsal and ventral ventral, then the original mouth- 

 opening of the oesophagus must be looked for on the ventral surface 

 of the vertebrate brain in the region of the pituitary body or hypo- 

 physis, and on the dorsal side the tube representing the oesophagus 

 must be continuous with a large cephalically dilated tube, which 

 ought to pass into a small canal, to run along the length of the body 

 and terminate in the anus. 



This is exactly what is found in the vertebrate, for the infun- 

 dibular tube passes into the third ventricle of the brain, which forms, 

 with the other ventricles of the brain, the large dilated cephalic 

 portion of the so-called nerve tube, and at the junction of the medulla 

 oblongata and spinal cord, this dilated anterior part passes into the 

 small, straight, central canal of the spinal cord, which in the embryo 

 terminates in the anus by way of the neurenteric canal. If the 

 animal is regarded as not having been turned over, then the con- 

 clusion that the infundibulum was the original oesophagus leads 

 immediately to the further conclusion that the ventricles of the verte- 

 brate brain represent the original cephalic stomach, and the central 

 canal of the spinal cord the straight intestine of the arthropod ancestor. 



