228 Mr Kerr, Note on Hypotheses as to the Origin 



(2) the skeleton of the paired fins with their limb girdle 

 are derived by modification from the gill rays attached to a 

 branchial arch, and hence, by implication, the paired fins them- 

 selves are derived from the septa separating adjacent gill-clefts. 



Balfour Vieiv. 



(1) This view as at present held was based by Balfour on 

 his observation that in the embryos of certain Elasmobranchsthe 

 rudiments of the pectoral and pelvic fins are at a very early period 

 connected together by a longitudinal ridge of thickened epiblast — 

 of which indeed they are but exaggerations. In Balfour's own 

 words referring to these observations: " If the account just given 

 of the development of the limb is an accurate record of what 

 really takes place, it is not possible to deny that some light is 

 thrown by it upon the first origin of the vertebrate limbs. The 

 facts can only bear one interpretation-, viz.,, that the limbs are the 

 remnants of continuous lateral fins." 



A similar view to that of Balfour was enunciated almost 

 synchronously by Thacher and a little later by Mivart — in each 

 case based on anatomical investigation of Selachians — mainly 

 relating to the remarkable similarity of the skeletal arrangements 

 in the paired and unpaired fins. 



(2) The Gegenbaur view is based upon the skeletal struc- 

 tures within the fin. 



(a) According to this the skeletal arrangements in all 

 Vertebrate limbs are modifications of a primitive form — the biserial 

 Archipterygium, which persists in a practically unmodified con- 

 dition in the paired fin of Geratodus. 



(/3) The biserial archipterygium with its limb girdle is in 

 turn derived from a series of gill-rays attached to a branchial 

 arch. 



The Gegenbaur theory of the Morphology of Vertebrate limbs 

 thus consists of two very distinct portions. The first, that the 

 archipterygium is the ground-form from which all other forms of 

 presently existing fin skeletons are derived, concerns us only 

 indirectly as we are dealing here only with the origin of the limbs, 

 i.e., their origin from other structures that were not limbs. 



It is the second part of the view that we have to do with, that 

 deriving the skeleton of the archipterygium, of the primitive 

 paired fin, from a series of gill rays, and involving the idea that the 

 limb itself is derived from the septum between two gill clefts. 



This theory rests upon (1) the assumption that the archiptery- 

 gium is the primitive type of fin, and (2) the fact that amongst the 

 Selachians is found a tendency for one branchial ray to become 



