444 THE ORIGIN OF VERTEBRATES 



roots. Lwoff and Eolph both describe a direct communication 

 between the spinal cord and the notochord by means of fibres 

 passing through these holes, without however looking upon this con- 

 nection as a nervous one. Joseph alone asserts that no absolute 

 connection exists, for the internal elastic layer of the notochord, 

 according to him, is not interrupted at these holes, and forms, therefore, 

 a barrier between the fibres from the spinal cord and those from the 

 interior of the notochord. Still, whatever is the ultimate verdict as 

 to these fibres, the suggestive fact remains of the spaces in the 

 notochordal sheath and of the corresponding projecting root-like fibres 

 from the spinal cord. The whole appearance gives the impression of 

 some former connection, or rather series of connections, between the 

 spinal cord and the notochord, such as would have occurred if nerves 

 had once passed into the notochord. On the other hand, such nerves 

 were not arranged segmentally with the myotomes, for, according to 

 Joseph, in the middle of the animal ten to twelve such holes occur in 

 one body-segment. In Apus the appendages are more numerous than 

 the body-segments, so that it is not necessary for a segmental arrauge- 

 ment to coincide with that of the body-segments. 



The Origin of the Alimentary Canal. 



In close connection with the notochord is the alimentary canal. 

 Any explanation of the one must be of assistance in explaining 

 the other. 



According to the prevalent embryological teaching, the body is 

 formed of three layers, epiblast, hypoblast, and mesoblast, and the 

 gastrtea theory of the origin of all Metazoa implies of necessity that 

 the formation of every individual commences with the formation of 

 the gut. For this reason the alimentary canal must in every case 

 be regarded as the earliest formed organ, however late in the develop- 

 ment it may attain its finished appearance. Hence the notochord is 

 spoken of as developed from the mid-dorsal wall of the alimentary 

 canal. It is possible to look at the question the other way round, 

 and suppose that the organ whose development is finished first is 

 older than the one still in process of making. In this case it would 

 be more right to say a ventral extension of the tissue, which gives 

 rise to the notochord, takes place and forms the alimentary canal. 

 It is, to my mind, perfectly possible, and indeed probable, that 



