484 THE ORIGIN OF VERTEBRATES 



Where the food is not, there will be no gut formation, whatever may 

 have been the previous history of that layer. If, then, we suppose, 

 as I do, that the vertebrate arose from a scorpion-like animal without 

 any reversal of dorsal and ventral surfaces, and that the central 

 nervous system remained the same in the two animals, then the 

 comparison of the development of the two embryos shows that the 

 one would be derived from the other if the yolk-mass shifted from 

 the dorsal to the ventral side of the nervous system. This would 

 leave the dorsal epithelial layer of the original syncytium free from 

 pabulum ; it would no longer form the definite gut, but it would 

 still tend to form itself in the same manner as before, would still grow 

 from a vcntrally situated germ-band dorsalwards to form a tube, ivoidd 

 recapitulate its fast history, and show how the alimentary canal of the 

 arthropod became the neural canal of the vertebrate. Although this 

 alimentary canal is formed in the same way as before, it is no longer 

 recognized as homologous with the scorpion's alimentary canal, but 

 because it no longer absorbs pabulum, and does not therefore form 

 the definite gut, it is called an epiblastic tube, and, in the words of 

 Hay Lankester, has no developmental importance. 



All the arthropods are built up on the same type, and in all the 

 development may in its broad outlines be referred to the type just 

 mentioned. So also with the vertebrate group; in both cases the 

 position of the central nervous system determines the starting area 

 of embryonic growth. In both cases the absorbing layer shows the 

 position of the definite gut. A concentrated nervous system of this 

 type is common to all the segmented animals from the annelids to 

 the vertebrates, and in all cases the germ-band which indicates the 

 first formation of the embryo is in the position of this nervous system. 



As far as the embryo is concerned, there is no great difficulty in 

 the conception that the yolk-mass may have shifted from one side to 

 the other in passing from the arthropod to the vertebrate, for in the 

 arthropod the embryo at first is surrounded by yolk and then passes 

 to the periphery of the egg. If it is permissible to speak of a dorsal 

 and ventral surface to an egg, and we may imagine the egg held with 

 such dorsal surface uppermost, then the yolk would be situated 

 ventrally to the embryo, as in the vertebrate, if the protoplasmic 

 cells of the embryo rose from their central position to the surface 

 through the yolk, while if they sank through the yolk, the yolk 

 would be situated dorsally to the embryo, as in the arthropod. 



