540 POPULAR SCIENCE MONTHLY. 



appears as 'a little papilla and not as a fold, where the body walls join 

 the hinder upper portion of the yolk-sac, a very little way in front of the 

 vent. ' ' These two modes of origin, ' observes Dr. Eyder, ' are therefore 

 in striking contrast and well calculated to impress us with the protean 

 character of the means at the disposal of Nature to achieve one and the 

 same end.' 



Current Tlieories. 



There are three chief theories as to the morphology and origin of 

 the paired fins : 



The earliest is that of Gegenbaur, supported by numerous others, 

 tli at these fins are derived from modified gill-arches or septa between the 

 gill-openings. According to this theory, the skeletal arrangements of 

 the vertebrate limb are derived from modifications of one primitive 

 form, a structure made up of successive joints, with a series of fin-rays 

 on each side of it. To this structure, Gegenbaur gives the name of 

 archipterygium. It is found in the shark, Pleuracanthus, and in all 

 the Dipnoan and Crossopterygian fishes, its primitive form being still 

 retained in the Australian genus of Dipnoans, Neoceratodus. This 

 biserial archipterygium with its limb girdle is derived from a series of 

 gill rays attached to a branchial arch. 



Professor J. Graham Kerr observes : 



' ' The Gegenbaur theory of the morphology of vertebrate limbs thus 

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

 gium is the ground-form from which all other forms of presently exist- 

 ing fin skeletons are derived, concerns us only indirectly as we are deal- 

 ing 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 archipterygium, the skeleton 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 view is based on the skeletal structures within the fin. It 

 rests upon: (1) The assumption that the archipterygium is the primi- 

 tive type of fin, and (2) the fact that amongst the Selachians is found 

 a tendency for one branchial ray to become larger than the others, and 

 when this has happened, for the base of attachment of neighboring 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 iden- 

 tical with the archipterygium. Gegenbaur suggests that the archiptery- 

 gium actually has arisen in this way in phylogeny. 



The theory of Balfour, adopted by Dohrn, Wiedersheim, Thacher, 



