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 
stil tend to form itself in the same manner as before, would still grow 
from a ventrally situated germ-band dorsalwards to form a tube, would 
recapitulate its past history, and show how the alimentary canal of the 
arthropod became the newral 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 
Ray 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 ege 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. 
