LINNEAN SOCIETY OF LONDON. 27 



Dr. H. Gadow, F.R.S. (Visitor), followed, and said : — When 

 Dr. Graskell explained his hypothesis at a meeting of the Cambridge 

 Philosophical Societ_y, fourteen years ago, I was the only one who 

 had the courage of pleading for its being given a chance. It has 

 survived pitiful contempt and ridicule. 



If we want to join the ends of a broken chain, we must be 

 clear about the links. I propose pointing out the last Vertebrate 

 link, by reconstructing an early Vertebrate analytically. 



Ever since (jegenbaur based his investigations into the compo- 

 sition of the cranium upon Elasmobranchs, and as since, after him, 

 Balfour discovered so many important features in their embryonic 

 development, the Elasmobranchs have come to be looked upon as 

 the ideally lowest typical Vertebrates. Dohrn even went so far 

 as to explain the Cyclostomes out of the way of direct ancestry as 

 degenerated Elasmobranchs. 



This Elasmobranch worship is wrong. They are a side-branch 

 which leads to nothing. The main stem of the Vertebrate 

 descent passes through what we may call Gano-Dipnoi, and their 

 ancestors, Proto-Gano-Dipnoi, presumably were still devoid of 

 paired limbs, and still lower down were not yet Gnathostomes. 

 We can reconstruct further: With a mouth not terminal but 

 ventral : their bulk consisting of a large anterior complex and a 

 short, tapering tail, both segmented and metameric. Condensation 

 and fusion produced a head which was so large because it con- 

 tained all the principal organic systems, as nervous, digestive, 

 respiratory, vascular, and possibly excretory and generative. 



Metamerism in this anterior complex, the incipient head, was 

 doomed, but in the posterior portion it underwent renewed 

 activity. Not only were more segments formed by interstitial 

 budding, but metamerism ran wild, culminating, besides other 

 features, in vertebralization. 



The latter proceeded from the tail end forwards, and it is idle 

 to seek for vertebrae in the primitive bead, excepting in the part 

 from the vagus backwards, which in the early creature we are 

 dealing with, was a very I'ecent formation. 



Meanwhile, the posterior or tail portion becoming larger, part 

 of it, from before backwards, was converted into a trunk, as this 

 was receiving most of those organs which were crowded out from 

 the consolidating head, and also no doubt owing to the repetitional 

 budding backwards of some of these organs. Thus we have 

 arrived at a Tadpole-shaped A^ertebrate of which some Ostraco- 

 dermi with their vertebralized tails are not a bad sample. 



Gegenbaur had taught us to consider the spinal cord as an 

 outgrowth from the older brain. The greater part of the chorda 

 is likewise due to a secondary growth backwards, this organ not 

 being laid down in its totality, certainly not in the tail where it 

 ought to have arisen if originally intended for an axial stiffening 

 organ. It arises, however, in the trunk, and since this is a later 



