Morphology of the Fins 



graphs in quotation-marks being taken from his paper. We 

 may first consider Balfour's theory of the lateral fold. 



Balfour's Theory of the Lateral Fold. "The evidence in 

 regard to this view may be classed under three heads, as onto- 

 genetic, comparative anatomical, and paleontological. The 

 ultimate fact on which it was founded was Balfour's discovery 

 that in certain Elasmobranch embryos, but especially in Tor- 

 pedo (Narcobatis), 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 Mollier, a supporter 

 of the lateral-fold view, is to the effect that it does not occur 

 in such ordinary sharks as Pristiurus and Mustelus, while it is 

 to be gathered from Balfour himself that it does not occur 

 in Scyllium (Scyliorhinus] . 



" 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 corre- 

 lation with their peculiar 

 mode of life, the paired fins 

 have undergone (in secondary 

 development) enormous ex- 

 tension along the sides of 

 the body, and their continu- 

 ity in the embryo may well 

 be a mere foreshadowing of 

 this. 



"An apparently powerful support from the side of embry- 

 ology came in Dohrn and Rabl's 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 farther back. As either explanation seems 

 an adequate one, I do not think we can lay stress upon this 

 body of facts as supporting either one view or the other. The 



FIG. 59. Arm of a frog. 



