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, 
TuE 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 
