12 THE ANCESTRY OF VERTEBRATES 



the whole intestinal tract of the latter, from the mouth to 

 the anus, pass into the neural tube of the former, the 

 stomach giving rise to the brain and the intestine to the 

 spinal canal, while the infundibulum marks the place of 

 the old mouth. A new alimentary canal to replace the lost 

 one is formed from a median groove of the ventral body wall 

 which by the growing out of two lateral folds is converted 

 into a tube. Of course Gaskell is engaged in grave conflict 

 with the doctrine of the germinal layers — the central nerv- 

 ous system of Vertebrates would be of entodermal ^), the 

 gut of ectodermal origin! — and with the other principles 

 of embryology. GASKELL truly tries to show, that his theory 

 does not contravene these principles, but he can do so only 

 by expressing as his opinion, that "the cmbryologists have 

 to a large extent gone wrong in their fundamental princi- 

 ples" (p. 459) and by rejecting the doctrine of the homology 

 of the two primary germinal layers in Metazoa. I need not 

 emphasize, that my theory on the contrary is in perfect 

 accordance with the doctrine of the germinal layers; as is 

 the general rule in the animal kingdom the nervous system 

 in Chordates is of ectodermal origin. Almost the only point 

 in which 1 agree with GASKELL is "that the clue to the 

 origin of vertebrates is to be found in the tubular nature 

 of the central nervous system of the vertebrate"; however, 

 Gaskell and the writer differ widely in their application 

 of this principle 



To the question as to what may have been the cause 

 of such a remarkable change of function as is the conver- 

 sion of the stomodaeum of the Protostomia into the neural 

 tube of the Chordates, the answer is not easily given. Some 

 reflections perhaps will assist in making us a little more 

 familiar with the idea. 



Conversion of stomodaeum into medullary tube. — In the first 

 place the question can be asked : what is the original 

 significance and function of the stomodaeum? Why did not 

 the blastopore, the "Urmund", as e. g. in hydroid polyps, 

 remain also in higher forms the definitive mouth. Why 

 was an ectodermal entrance to the gut formed in connection 

 to it? Partly no doubt this finds its explanation in the 

 dense coat of cilia investing the inner surface of the stomo- 



') In part at least, for according to Gaskell the ganglia of the 

 Arthropod ancestors have applied themselves to and enclosed the 

 alimentary canal, forming together with it the neural tube. 



