PAIRED FINS 73 



especially in front of the fins. Concrescence, the second factor, 

 may introduce profound modification (Thacher [434], Mivart [300]). 

 Not only do the skeletal radials often fuse, forming basal plates or 

 jointed axes of most varied shapes (Figs. 48-50, 52-54), but the 

 muscular segments also possibly mix to some extent in ontogeny, 

 and lose their primitive metamerism. 



The very important subject of the origin of the paired limbs of 

 Gnathostomes must now be discussed. As it is generally agreed 

 that the primitive form of these must have been the fin-like 

 ' ichthyopterygium,' the evolution of the fish fin only need for the 

 present be considered. What appear to be the less modified fish 

 possess a pair of pectoral fins supported by a pectoral girdle behind 

 the gills, and a pair of pelvic fins supported by a pelvic girdle 

 passing just in front of the anus. 



According to one theory, that of Gegenbaur [155, 157, 162], 

 the paired limbs are modified gill-structures ; the girdles represent- 

 ing the gill-arches, and the fin-fold and the fin-skeleton representing 

 the gill-flap and its gill-rays. The position of the fins far back, 

 especially of the pelvic fins, is explained as due to the shifting 

 backwards or migration of these posterior arches, which have lost 

 their original branchial function. This may shortly be called the 

 gill-arch theory. 



A second and rival theory, that of Balfour [27-29], Thacher 

 [434], and Mivart [300], holds that the paired fins are of the same 

 nature as the median fins, and have been developed from paired 

 longitudinal lateral fin-folds ; the somactidia, or endoskeletal radials, 

 would, in both cases, have arisen for the stiffening of the fin-folds. 

 The girdles would have been developed by the extension inwards 

 of these rays so as to afford a firm basal support to the fins. This, 

 the most generally accepted view, is known as the lateral-fold theory. 



The paired fins develop, on the whole, just like the unpaired 

 fins. They appear as longitudinal folds of the body-wall into which 

 grows mesenchymatous mesoderm. Muscle-buds push their way 

 into the fin-fold from the neighbouring myotomes (Dohrn [118], Rabl 

 [336, 338], Mollier [301-302]), two growing out of the lower end 

 of each myotome in the fin region in Elasmobranchs (Fig. 47), or 

 one from each myotome in other forms. The buds spread outwards, 

 dividing into upper and lower halves, which form the dorsal and 

 ventral radial muscles of the adult fin. Endoskeletal radials, 

 somactidia, are differentiated between the upper and lower muscle- 

 buds. At first the girdle, basals, and peripheral radials appear as 

 a continuous rudiment of pro-cartilaginous cells. Later on the 

 individual elements seen in the adult arise as separate chondrifica- 

 tions, leaving non-cartilaginous joints (Balfour [28], Mollier [301], 

 Ruge [377a]). 



