NOTES ON MF,DIAN AND PAIRED FINS OP FISH. 347 



The exact origin of tlie muscles of the paired fins is rarely 

 as easily traceable as in the Elasniobranch ; but, as far as is 

 known, the above rule holds good for all Grnathostomes. 

 Unfortunately, in many forms distinct muscle-buds are not 

 produced, and the muscle-producing cells are budded off 

 separately from the Myotomes. Nevertheless, the derivation 

 of the limb muscles has been distinctly traced in the case of 

 vainous Elasmobranchs, of Salmo (Harrison 19), of Acipenser 

 (Mollier 26), Cyclopterus (Guitel 17), aud of Lacerta 

 (Mollier26). In all cases where the development has been 

 followed it has been shown that the nerve-supply ("limb- 

 plexus") in the adult is a sure guide to the identifica- 

 tion of the segments from which the muscles have 

 been derived. Segments before, and behind, those of the 

 limb-plexus may have ceased to contribute, owing to reduc- 

 tion during development, but adult nerve-supply shows which 

 segments have contributed most. 



Unfortunately, with regnrd to the skeletal element the 

 facts are not so well established. From the very nature of 

 the case, it is much more difficult to deal Avith. The carti- 

 laginous radials are merely local differentiations in continuous 

 connective tissue, or mesenchyme. And although probably 

 this tissue has itself been derived from segmental sclerotomes, 

 yet the limits of the segments have long ceased to exist when 

 the radials develop. 'J'here is, however, no valid reason for 

 believing that radials are less constant than the muscles 

 with which they are related. Nor is there any evidence that 

 the skeleton of the pelvic limb, for instance, is formed of 

 tissue derived from any other segments but those belonging 

 to its muscles. 



Of course, limb elements may undergo relative displace- 

 ment in the course of ontogeny. In the development of fins 

 the anterior muscle-buds are relatively disj^laced back^vards, 

 and the posterior buds are relatively displaced forwards — 

 this is the jorocess of concentration. It may also happen, 

 in the higher vertebrates, that a limb may be shifted a 

 segment or two up, or down, the vertebral column with which 



