Morphology of the Fins 67 



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

 gro\\rth proceeds, its base does not expand antero-posteriorly, 

 but tends rather to become narrowed, so that it has a peduncu- 

 lated 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 notice- 

 ably within the inner contour-line. The free border of the fin- 

 fold grows out laterally and longitudinally, expanding the por- 

 tion outside of the inner contour-line of the fin into a 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 appears as " a little papilla and 

 not as a fold, where the body-walls join the hinder upper por- 

 tion of the yolk-sac, a very little way in front of the vent." 

 " These two modes of origin," observes Dr. Ryder, " are therefore 

 in striking contrast and well calculated to impress us with the 

 protean character of the means at the disposal of Nature to 

 achieve one and the same end." 



Current Theories as to Origin of Paired Fins. — There are three 

 chief theories as to the morphology and origin of the paired fins. 

 The earliest is that of Dr. Karl Gegenbaur, supported by 

 various workers among his students and colleagues. In his view 

 the pectoral and ventral fins are derived from modifications of 

 primitive gill-arches. According to this theory, the skeletal 

 arrangements of the vertebrate limb are derived from modifica- 

 tions of one primitive form, a structure made up of successive 

 joints, with a series of fin-rays on one or both sides of it. To 

 this structure Gegenbaur gives the name of archipterygium. 

 It is found in the shark, Pleiiracantlnis, in Cladodus, and in 

 all the Dipnoan and Crossopterygian fishes, its primitive form 

 being still retained in the Australian genus of Dipnoans, Neocera- 

 todus. This biserial archipterygium with its limb-girdle is 



