[35] THE EVOLUTION OF THE FINS OF FISHES. 1015 



fibrous rays of the protopterygian stage terminate proxinially are in all 

 probability homologous for reasons which will presently be given. The 

 theory that certain portions of the primitive median tin-folds degenerate 

 in those forms which have intervals between the permanent fins, as in- 

 sisted upon in another part of this paper, is now conclusively demon- 

 strated by the fact that in the i)reanal fin-fold of the embryo Salmon, 

 just after hatching, we find the fin-fibers of the protopterygian stage 

 present, though this fin never develops, but is gradually lost, nearly 

 vanishing after about four weeks. This fact indicates that the Salmon 

 has descended from a form which had an unpaired fin in front of the 

 vent, and that the development of rays went no farther in this fin than 

 the formation of the two series of fin-fibers. I have not found any evi- 

 dence amongst recent fishes, of the existence of preanal fins, though 

 Balistes seems to have the ventral tins coalesced into what might at first 

 be taken for a preanal fin, were it not that its rays are alternately ele- 

 ments of the pelvic fins of opposite, sides, as shown by their close rela- 

 tion anteriorly to the falcate and greatly prolonged pubic bones. 



The larvse of Ghondrostei are represented as having preanal fin-folds 

 with embryonic rays, but the Elasmobranch larv 86 which I have been 

 able to examine do not seem to possess them. Inasmuch, therefore, as 

 they are not constantly present in the larvae of all fishes it is probably 

 premature to insist that they indicate anything more than this, namely, 

 that the preanal fin-folds have been inherited from forms in which they 

 were functional 5 what these forms were we do not know. 



The pectorals of young Rays (? B. stellata) one and a half inches lon^ 

 do not seem to contain any horny fibers, though they are abundant 

 in all of the fins of young Dog-fishes {Squalus), considerably smaller, 

 where they are present as very short fibers in the marginal part of the 

 fins beyond the point to which the cartilage protrudes. In young Lam- 

 preys four inches long I have found no traces of them, the vertical fins 

 in these being supported by branching rays consisting entirely of carti- 

 lage which extend nearly to the margin of the fin-fold. In i3repared sec- 

 tions of Tadpoles three-eighths of an inch long no horny fibers are ap- 

 parent in the median fins. The Amphibia and Marsipobranchs are ac- 

 cordingly excluded from that category, the larval stages of which are 

 characterized by the possession of horny fibers in the fin-folds, since a 

 protopterygian stage is found only in Dipnoi, Ghondrostei, Elasmohran- 

 ehii, Holocephali, and Teleostei. 



The Leptocardii are also excluded from the above assemblage of forms 

 since the development of their so-called fin-rays is entirely different 

 from that of any of the branchiferous Vertebrates possessing fin-rays, 

 according to the account given of their development by Kowalevsky.* 

 They arise in this form from aggregations of cells which become thrust 

 apart centrally by the appearance in their midst of a vacuole filled with 



* Entwickelungesohichte des Ampliioxus lanceolatus. M6m. I'Acad. imp. des sci- 

 ences de St.-P6tersbourg, YJI^ s6r., t. XI, No. 4. (See especially p. 12, PI. Ill, Fig. 39.) 



