NOTES ON MEDIAN AND PAIRED FINS OF FISH. 371 



selves so readily to experiment. I have not yet been able to 

 apply so decisive a set of tests to the delicate "plexus" sup- 

 plying these fins. But it can be easily shown that the suc- 

 cessive stimulation of the rami dorsales sharing in the 

 " plexus " induces the successive contraction of the series of 

 corresponding radial muscles. 



It results from these experiments that the metamerism of 

 the fin elements may remain undisturbed in the paired, and 

 probably also in the unpaired, fins of Elasmobranchs. In 

 this they agree with the evidence of embryology. 



Summary and Conclusion. 



The chief observations described above may be summarised 

 as follows : The development of the median dorsal fins is 

 essentially similar to that of the paired fins. They arise as 

 longitudinal folds, into which grow buds from the myotomes. 

 Some fourteen or sixteen myotomes contribute to the fin each 

 one muscle-bud. Concentration sets in almost from the first 

 appearance of the buds ; it is chiefly, if not entirely, due 

 to the body growing faster than the fin. Along the greater 

 part of the dorsal fin each muscle-bud becomes converted 

 into one radial muscle. At the extreme ends of the fins the 

 exact metameric origin of the muscles is difficult to trace and 

 is somewhat obscured. Only here fusion of neighbouring 

 segmental buds perhaps takes place. At certain stages slender 

 strands of embryonic tissue connect the bases of the radial 

 muscles; these are probably rudiments of the nerve-plexus. 

 Neither the study of development nor of the adult structure 

 affords any definite evidence that the primitive metamerism 

 of the musculature is lost. Experiments seem to establish 

 that the radial muscles remain haploneurous, retaining their 

 primitive connection with the nerve belonging to that myotome 

 from which they have been developed. The nerve- " plexus " 

 of the fins is composed of intertwining sensory fibres, along 

 or through which the motor fibres proceed to their destina- 

 tion without mixino' with those of other segments. There is 



