434 THE ORIGIN OF VERTEBRATES 



vertebrate there is nothing but a gap in the invertebrate, but the 

 position of that gap can be settled with great accuracy from 

 the previous comparison of organs in the two groups. So, also, the 

 alimentary canal of the vertebrate is from the very nature of the 

 case a new organ, yet, as has been shown in Chapter V., the com- 

 parison of the respiratory organs in the two groups gives a strong 

 suggestion of the manner in which such a canal was formed. 



'oo v 



The Origin of the Notochord. 



The time has now come to endeavour to frame a plausible theory 

 of the method of formation of the notochord and the new alimentary 

 canal, and thus to complete the diagram on p. 413. The comparative 

 method is no longer available, for these structures are both unrepre- 

 sented as such in the arthropod ; any suggested explanation, therefore, 

 must be more tentative, and cannot give the same feeling of certainty 

 as is the case with all the organs already considered. Our only chance 

 of finding out the past history of the notochord lies in the embryo- 

 logical method, in the hope that, according to the ' law of recapitu- 

 lation,' the ancestral history may be repeated in the ontogeny with 

 sufficient clearness to enable some conclusion to be drawn. 



At the outset, one point comes out clearly— the close relationship 

 between the notochord and the vertebrate gut ; they are both derived 

 from the same layer, both parts of the same structure. On this 

 point all embryologists are agreed ; it is expressed in such statements 

 as, "the notochord, as well as the alimentary canal, is formed from 

 hypoblast " ; " the notochord arises as a thickening in the dorsal wall 

 of the alimentary canal." The two structures are so closely connected 

 together that they must be considered together. If we can conjecture 

 the origin of the one, we may be sure that we have the clue to the 

 origin of the other. The two together form the one new organ which 

 distinguishes the vertebrate from the arthropod, the only thing left 

 which requires explanation for the completion of this strange history. 



What, then, is the notochord ? What are its characteristics ? In 

 the highest vertebrates it is conspicuous only in the embryo ; with 

 the development of the axial skeleton it is more and more squeezed 

 out of existence, until in the adult it is no longer visible. By the 

 ' law of recapitulation ' this developmental history implies that, as we 

 descend the vertebrate phylum, the notochord ought to be more and 



