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 Theories.
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, that 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 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 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 primitive 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 identical with the archipterygium. Gegenbaur suggests that the archipterygium actually has arisen in this way in phylogeny.
The theory of Balfour, adopted by Dohrn, Wiedersheim, Thacher,
