4 THE ORIGIN OF VERTEBRATES 



both cranial and spinal, and also the original segmental character of 

 this part of the nervous system. 



I could not, therefore, help being struck by the force of the 

 comparison between the central nervous systems of Vertebrata and 

 Appendiculata as put forward again and again by the past gene- 

 • ration of comparative anatomists, and wondered why it had been 

 discredited. There in the infundibulum was the old oesophagus, 

 there in the cranial segmental nerves the infracesophageal ganglia, 

 there in the cerebral hemispheres and optic and olfactory nerves the 

 supracesophageal ganglia, there in the spinal cord the ventral chain 

 of ganglia. But if the infundibulum was the old oesophagus, what 

 then ? The old oesophagus was continuous with and led into the 

 cephalic stomach. What about the infundibulum ? It was continuous 

 with and led into the ventricles of the brain, and the whole thing 

 became clear. The ventricles of the brain were the old cephalic 

 stomach, and the canal of the spinal cord the long straight intestine 

 which led originally to the anus, and still in the vertebrate embryo 

 opens out into the anus. Not having been educated in a morpho- 

 logical laboratory and taught that the one organ which is homologous 

 throughout the animal kingdom is the gut, and that therefore the 

 Efut of the invertebrate ancestor must continue on as the gut of 

 the vertebrate, the conception that the central nervous system has 

 grown round and enclosed the original ancestral gut, and that the 

 vertebrate has formed a new gut did not seem to me so impossible 

 as to prevent my taking it as a working hypothesis, and seeing to 

 what it would lead. 



This theory that the so-called central nervous system of the 

 vertebrate is in reality composed of two separate parts, of which 

 the one, the segmented part, corresponds to the central nervous 

 system of the highest invertebrates, while the other, the unseg- 

 mented tube, was originally the alimentary canal of that same 

 invertebrate, came into my mind in the year 1887. The following 

 year, on June 23, 1888, I read a paper on the subject before the 

 Anatomical Society at Cambridge, which was published in the Journal 

 of Anatomy and Physiology, vol. 23, and more fully in the Journal of 

 Physiology, vol. 10. Since that time I have been engaged in testing 

 the theory in every possible way, and have published the results of 

 my investigations in a series of papers in different journals, a list of 

 which I append at the end of this introductory chapter. 



