190 THE POPULAR SCIENCE MONTHLY. 



ing around the tip of the tail forward to the vent, thence forward ta 

 about the middle of the body. Such a primordial median tin exists 

 in the young of all fishes whose development has been studied. The 

 permanent fins seem to result from the formation of cartilaginous or 

 bony rays either througliout most or the whole of its length, as with 

 lampreys and common eels; or at several points, as with the macke- 

 rel and Polypterus ; or at three, or two, or only one, as with the cod, 

 the blue-fish, and the pickerel. The intervening portions disappear. 

 The hinder part of the primordial fin of the smallest gars presents 

 four points of darker coloration, two above and two below. The 

 posterior upper spot presents no rays, and later seems to disappeai-.' 

 The other three are evidently the beginnings of fins. The anterior 

 above and that below occupy the positions of the future dorsal and 

 anal fins. The destiny of the hinder lower spot is better seen by 

 comparison with larger examples. 



The series given in Figs. 8 and 9 shows that, as the gar increases 

 in length, the primordial fin disappears, the dorsal and anal increase, 

 and the end of the tail becomes more slender and pointed. But the 

 most striking change consists in the enlargement of the hinder lower 

 spot into what may be called the infra-caudal lobe. The rays of this 

 become longer and more numerous. They project beyond the margin 

 of the primordial fin, so as to leave a decided notch, as in Fig. 8, A. 



In Fig. 8, B, the end of the body merits the name of filament, and 

 the relative size of it and the lobe is reversed. Ai'terward, partly by 

 more rapid increase of the lobe, and partly by absorption of the fila- 

 ment, the latter seems smaller and smaller, and at last disappears; so 

 that the lobe, from having been at first an outgrowth from the fila- 

 ment, finally becomes the whole of the tail or caudal fin. 



It appears, then, that the hinder end of the body undergoes con- 

 siderable change before reacliing the adult condition. Aside from the 

 partial disappearance of the primordial median fin and the gradual 

 development of the ventrals, the dorsals, and the anal, the caudal fin 

 assumes at least three distinct forms. The first is lance-shaped and 

 simple, like that of Amphioxits, the eel, the lamprey, salamanders, and 

 tadpoles. The second is compound, with a slender filament above and 

 a broader fin below, as with some sturgeons and sharks. The third 

 consists entirely of the lower fin, which is enlarged and brought into a 

 direct line with the body, the longest rays being a little above the 

 middle. Its upper and lower borders are now thicker and stronger 

 than the intermediate portions ; whereas in the first stage the carti- 

 lage and muscle are in the centre, the upper and lower borders being 

 very thin. 



In short, the tail of the gar-pike undergoes a decided transforma- 

 tion. And one naturally inqnires, " What is the occas\on for it ? " 



' It may have a morphological significance, as suggested further on respecting the fossil 

 Glyptolcemv^. 



