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 cesophagus, 
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 cesophagus, what 
then? The old cesophagus 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 
gut 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. 
