ORIGIN OF THE FINS OF FISHES. 



539 



vertebrates, arise locally, not, however, as blunt processes but as short 

 longitudinal folds, with perhaps a few exceptions. The pectorals of 

 Lepisosteus originate in the same way. Of the paired fins, the pectoral 

 or anterior pair seems to be the first to be developed, the ventral or 

 pelvic pair often making its appearance until after the absorption 

 of the yolk-sac has been completed, in other cases, before that event as in 

 Salmo and in Gambusia. The ventral undergoes less alteration of 

 position during its evolution than the posterior pair." 



In the codfish (Gadus callarias) the pectoral fin-fold "appears as 

 a slight longitudinal elevation of the skin on either side of the body nf 

 the embryo a little way behind the auditory vesicles, and shortly after 

 the tail of the embryo begins to bud out. At the very first, it appears 

 to be merely a dermal fold, and in some forms, a layer of cells extends 

 out underneath it from the sides of the body but does not ascend into it. 

 It begins to develop as a very low fold, hardly noticeable, and as growth 

 proceeds, its base does not expand antero-posteriorly but tends rather 

 to become narrowed, so that it has a pedunculated form. With the 

 progress of this process the margin of the fin-fold also becomes thinner 

 at its distal border, and at the 

 basal part mesodermal cells 

 make their appearance more 

 noticeably within the inner con- 

 tour line. In some species I am 

 quite assured that there is a 

 mesodermal tract or plate of 

 cells developed just behind the 

 auditory vesicles, just outside 

 the source of the mesodermal 

 cells which are carried up into 

 the pectoral fin-fold. This is 

 developed at about the time of 

 the closure of the blastoderm and these lateral mesodermal folds of tis- 

 sue may be called the pectoral plates. The free border of the fin-fold 

 grows out laterally and longitudinally, expanding the portion outside of 

 the inner contour line of the fin into fan-shape. This distal thinner 

 portion is at first without any evidence of rays ; further than that there 

 is a manifest tendency to a radial disposition of the histological elements 

 of the fin." 



The next point of interest is found in the change of position of the 

 pectoral fin by a rotation on its base. This is associated with changes 

 in the development of the fish itself. The ventral fin is also, in most 

 fishes, a short horizontal fold and just above the preanal part of the 

 median vertical fold which becomes anal, caudal and dorsal. But in 

 the top-minnow (Gambusia), of the order Haplomi, the ventral first 



Pectoral Fin of Hepterodontus philippi. 

 From Nature. 



