

of the Paired Limbs of Vertebrates. 229 



larger than the others, and when this has happened, for the base 

 of attachment of neighbouring rays to show a tendency to migrate 

 from the branchial arch on to the base of the larger or, as we may 

 call it, primary ray ; a condition coming about which were the 

 process to continue rather further than it is known to do in 

 actual fact would obviously result in a structure practically 

 identical with the archipterygium. 



Gegenbaur suggests that the Archipterygium actually has 

 arisen in this way in Phylogeny. 



Lateral fold tfiew. 



The evidence in regard to this view may be classified under 

 three heads as Ontogenetic, Comparative Anatomical, and Paleon- 

 tological. The ultimate fact on which it was founded was 

 Balfour's discovery that in certain Elasmobranch embryos, but 

 especially in Torpedo, the fin rudiments were at an early stage 

 connected by a ridge of Epiblast. I am not able to make out 

 what were the other forms in which Balfour found this ridge, but 

 subsequent research, in particular by M oilier, a supporter of the 

 lateral fold view, is to the effect that it does not occur in such 

 ordinary sharks as Pristiurus and Miistelus, while it is to be 

 gathered from Balfour himself that it does not occur in Scyllium. 



It appears to me that the knowledge we have now that the 

 longitudinal ridge is confined to the Rays and absent in the less 

 highly specialized sharks — greatly diminishes its security as a 

 basis on which to rest a theory. In the Rays in correlation with 

 their peculiar mode of life — the paired fins have undergone 

 enormous extension along the sides of the body 1 and their con- 

 tinuity in the embryo may well be a mere foreshadowing of this. 



An apparently powerful ally from the side of Embryology 

 came in Dohrn and Rabls' discoveries that in Pristiurus all the 

 interpterygial myotomes produce muscle buds. This however 

 was explained away by the Gegenbaur school as being merely 

 evidence of the backward migration of the hind limb — successive 

 myotomes being taken up and left behind again as the limb 

 moved further back. As either explanation seems an adequate 

 one, I do not think we can lay stress upon this body of facts 

 as evidence in favour of the lateral fold view. The facts of the 

 development of the skeleton can not be said to support the 

 fold view : according to it we should expect to find a series of 

 metameric supporting rays produced which later on become fused 

 at their bases. Instead of this we find a longitudinal bar of 

 cartilage developing quite continuously, the rays forming as pro- 

 jections from its outer side. 



1 I imagine that even supporters of the lateral fold view will admit this to be 

 secondary. 



