64 Morphology of the Fins 



of almost all fishes the same condition occurs. Dr. John A. 

 Ryder points out the fact that there are certain unexplained ex- 

 ceptions to this rule. The sea-horse, pipefish, and other highly 

 modified forms do not show this unbroken fold, and it is want- 

 ing in the embryo of the top-minnow, Gambnsia affims. Never- 

 theless 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 many, perhaps the majority of recent writers favor the 

 theory that these fins 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, at first 

 continuous, became soon atrophied in the middle, while at either 

 end it is highly specialized, at first into an organ of direction, 

 then into fan-shaped and later paddle-shaped organs of locomo- 

 tion. According to another view, the paired fins originated from 

 gill structures, originally both close behind the head, the ventral 

 fin migrating backward with the progress of evolution of the 

 species. 



Evidence of Paleontology. — If we had representations of all 

 the early forms of fishes arranged in proper sequence, we could 

 decide once for all, by evidence of paleontology, which form of 

 fin appears first and what is the order of appearance. As to 

 this, it is plain that we do not know the most primitive form 

 of fin. Sharks of unknown character must have existed long 

 before the earliest remains accessible to us. Hence the evidence 

 of paleontology seems confiicting and uncertain. On the whole 

 it lends most support to the fin-fold theory. In the later 

 Devonian, a shark, CladoselacJie fyleri, is found in which the 

 paired fins are lappet-shaped, so formed and placed as to suggest 



