Origin of the Median or Vertical Fins.
In the process of development the median or vertical fins are doubtless older than the paired fins or limbs, whatever be the origin of the latter. They arise in a dermal keel which is developed in a web fitting and accentuating the undulatory motion of the body. In the embryo of the fish the continuous vertical fin from the head along the back and around the tail precedes any trace of the paired fins.
In this elementary fin-fold slender supports, the rudiments of finrays, tend to appear at intervals. These are called by Eyder ray-hairs or actinotrichia. They are the prototype of fin-rays in the embryo fish, and doubtless similarly preceded the latter in geological time. In the development of fishes, the caudal fin becomes more and more the seat of propulsion. The fin-rays are strengthened, and their basal supports are more and more specialized.
That the vertical fins, dorsal, anal and caudal, have their origin in a median fold of the skin admits of no very serious question. In the lowest forms which bear fins these structures are dermal folds, being supported by very feeble rays. Doubtless, at first the vertical fins formed a continuous fold, extending around the tail, this fold ultimately broken by atrophy into distinct dorsal, anal and caudal fins. In the lower fishes, as in the earlier sharks, there is an approach to this condition of primitive continuity, and in the embryos of almost all fishes the same condition occurs. Dr. John A. Eyder points out the fact that there are certain unexplained exceptions to this rule. The sea-horse, pipe-fish and other highly modified forms do not show this unbroken fold, and it is wanting in the embryo of the top-minnow, Gambusia affinis. Nevertheless, the existence of a continuous vertical fold in the embryo is the rule, almost universal. The codfish with three dorsals, the Spanish mackerel with dorsal and anal finlets, the herring with one dorsal, the stickleback with a highly modified one, all show this character and we may well regard it as a certain trait of the primitive fish. This fold springs from the ectoblast or external series of cells in the embryo. The fin-rays and bony supports of the fins spring from the mesoblast or middle series of cells, being thrust upward from the skeleton as supports for the fin-fold.
Origin of the Paired Fins.
The question of the origin of the paired fins is much more difficult and is still far from settled, although the majority of recent writers have favored the theory that these are parts of a once continuous lateral fold of skin, corresponding to the vertical fold which forms the dorsal, anal and caudal. In this view the lateral fold is soon atrophied in the middle, while at either end it is highly specialized, at first into an organ
