The features of the development of the limbs just described, are especially well shewn in Torpedo; in the embryos of which the passage from the general linear thickening of epiblast into the but slightly better marked thickening of the thoracic fin is very gradual, and the fact of the limb being nothing else than a special development of the linear lateral thickening is proved in a most conclusive manner.
If the account just given of the development of the limbs 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.
The unpaired dorsal fin develops as a continuous thickening, which then grows up into a projecting fold of columnar cells. The greater part of this eventually atrophies, but three separate lobes are left which form the two dorsal fins and the upper lobe of the caudal fin.
The development of the limbs is almost identically similar to that of the dorsal fins. There appears a lateral linear thickening of epiblast, which however does not, like the similar thickening of the fins, grow into a distinct fold. Its development becomes confined to two special points, at each of which is formed a continuous elongated fold of columnar cells precisely like the fold of skin forming the dorsal fins. These two folds form the paired fins. If it be taken into consideration that the continuous lateral fin, of which the rudiment appears in Elasmobranchii, does not exist in any adult Vertebrate, and also that a continuous dorsal fin exists in many Fishes, the small differences in development between the paired fins and the dorsal fins will be seen to be exactly those which might have been anticipated beforehand. Whereas the continuous dorsal fin, which often persists in adult fishes, attains a considerable development before vanishing, the originally continuous lateral one has only a very ephemeral existence.
While the facts of development strongly favour a view which would regard the limbs as remnants of a primitively continuous lateral fin, there is nothing in the structure of the limbs of adult Fishes which is opposed to this view. Externally they closely resemble the unpaired fins, and both their position and nervous supply appear clearly to indicate that they do not belong to one special segment of the body. They appear rather to be connected with a varying number of segments; a fact which would receive a simple explanation on the hypothesis here adopted[198].
My researches throw no light on the nature of the skeletal parts of the limb, but the suggestion which has been made by Günther[199] with reference to the limb of Ceratodus (the most primitive known), that it is a modification of a series of parallel rays, would very well suit the view here proposed.
Dr Dohrn[200] in speaking of the limbs, points out the difficulties in the way of supposing that they can have originated de novo, and not by the modification of some pre-existing organ, and suggests that the limbs are modified gill-arches; a view similar to which has been hinted at by Professor Gegenbaur[201].
Dr Dohrn has not as yet given the grounds for his determination, so that any judgment on his views is premature.
None of my observations on Elasmobranchii lends any support to these views; but perhaps, while regarding the limbs as the remains of a continuous fin, it might be permissible to suppose that the pelvic and thoracic girdles are altered remnants of the skeletal parts of some of the gill-arches which have vanished in existing Vertebrates.
The absence of limbs in the Marsipobranchii and Amphioxus, for reasons already insisted upon by Dr Dohrn[202], cannot be used as an argument against limbs having existed in still more primitive Vertebrates.