44Q 



THE ORIGIN OF VERTEBRATES 



By. 



of the segmented animal, constitute the great peculiarity of the 

 vertebrate group ; it is not the unsegmented notochord alone which 

 requires explanation, but the presence of three such tubes in the 

 same animal. Any one of them might be the unsegmented gut of 

 the segmented animal. The most ventral tube is the actual gut of 

 the present vertebrate ; the most dorsal — the neural canal — was, 

 according to my view, the original gut of the invertebrate ancestor ; 

 the middle one — the notochordal tube — was, in all probability, also 

 once a gut, formed at the time when the exigencies of the situation 

 made it difficult for food to pass along the original gut. 



Yet another circumstance in favour of this suggestion is the very 



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Fig. 166. — Diagram to show the Meeting of the Four Tubes in such a 



Vertebrate as the Lamprey. 



Nc, neural canal with its infundibular termination ; Nch., notochord ; Al., alimentary 

 canal with its anterior diverticulum ; Hi/., hypophysial or nasal tube; Or., oral 

 chamber closed by septum. 



striking position of the anterior termination of the notochord. It 

 terminates at the point of convergence of three structures : — 



(1) The tube of the hypophysis or nasal tube. 



(2) The infundibulum or old mouth-termination. 



(3) The notochordal tube. 



To these may be added, according to Kupffer, in the embryonic 

 stage, the anterior diverticulum of the gut (Fig. 16B). 



This is a very significant point. Here originally, in the inverte- 

 brate stage, the olfactory passage opened into the old mouth and 

 oesophagus. Here, finally, in the completed vertebrate the same 

 olfactory passage opens into the new pharynx. In the stage between 

 the two it may well have opened into an intermediate gut, the noto- 

 chordal tube, its separation from which would leave the end of the 



