446 THE ORIGIN OF VERTEBRATES 



to that of the notochord, by the conversion of a groove into a tube. 

 Still more suggestive is it to find that the tube so formed has no 

 appearance whatever of segmentation ; it is as unsegmented as the 

 rest of the gut, although, as is seen in Fig. 62, the dorsal wall of 

 the respiratory chamber from which it arose is as markedly seg- 

 mented as any part of the animal. Here under our very eyes, in the 

 course of a few days or weeks, an object-lesson in the process of the 

 manufacture of an alimentary canal is carried out and completed, 

 and the teaching of that lesson is that a gut-tube may be formed 

 in the same way as the notochordal tube, by the conversion of a 

 grooved surface into a canal, and that gut-tube so formed, like the 

 notochord, loses all sign of segmentation, even although the original 

 grooved surface was markedly segmented. 



The suggestion then is, that the new gut may have been formed 

 by a repetition of the same process which had already given origin 

 to the notochord. 



Such a method of formation is not, in my opinion, opposed to the 

 evidence given by embryology, but in accordance with it ; the dis- 

 cussion of this point will come best in the next chapter, which treats 

 of the embryological evidence as a whole, and will therefore be left 

 till then. 



The Evidence given by the Innervation of the Vertebrate 



Alimentary Canal. 



Throughout this investigation the one fixed landmark to which all 

 other comparisons must be referred, is the central nervous system, and 

 the innervation of every organ has given the clue to the meaning of 

 that organ. So also it must be with the new alimentary canal ; by its 

 innervation we ought to obtain some insight into the manner of its 

 origination. In any organ the nerves which are specially of value in 

 determining its innervation, are of necessity the efferent or motor 

 nerves, for the limits of their distribution in the organ are much 

 more easily determined than those of the afferent or sensory nerves. 

 The question therefore of primary importance in endeavouring to 

 determine the nature of the origin of the alimentary canal from its 

 innervation is the determination of the efferent supply to the 

 musculature of its walls. 



Already in previous chapters a commencement has been made in 



