Ay. 
440 THE ORIGIN OF VERTEBRATES 
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 
Fic. 166.—D1aGRAM TO SHOW THE MEETING OF THE Four TUBES IN SUCH A 
VERTEBRATE AS THE LAMPREY. 
Ne., neural canal with its infundibular termination; Nch., notochord; Al., alimentary 
canal with its anterior diverticulum; Hy., 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. 166). 
This is a very significant point. Here originally, in the inverte- 
brate stage, the olfactory passage opened into the old mouth and 
cesophagus. 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 
Ne. 
Neh. 
Al. 
