NOTES ON MliDIAN AND PAIRED FINS OF FISH. oo3 



ontogeny of the dorsal fin of Acantliias (5). But this fin is 

 too much modified to yield much for our purpose. 



Develop m c n t of the Median Fins of S c y 1 1 i u m 



canicula. 



Fig. 1 is a careful reconstruction from longitudinal serial 

 sections of a portion of an embryo about 18 mm. long. 

 Unfortunately, this specimen was cut short at the forty-ninth 

 segment, so that only the first dorsal fin is included. 



At this stage is very well shown the origin of the nuiscle- 

 buds from the myotomes. One bud only is given off by each 

 myotome, not two as was surmised by Mayer. Already the 

 first steps in concentration are discernible in the convergence 

 of the buds towards a central region (about the forty-third 

 segment). The buds dwindle in size on both sides from this 

 region. They can be traced with certainty to the thirtj'- 

 second segment, and, more doubtfully, even beyond to the 

 twenty-eighth. There appear to be some eighteen buds in all. 

 The rudiment of the fin-fold itself, with its ridge of mesenchy- 

 matous tissue indicated by shading in the figure, extends over 

 at least a dozen segn}ents, passing off gradually in front. 



Fig. 25 is drawn with a camera from a specimen, 19 mm. 

 long, mounted Avholc in Canada balsam. The median fins are 

 here slightly more advanced, but only the largest muscle-buds 

 can be made out clearly on this preparation owing to the 

 smaller ones being hidden below the edge of the myotomes. 

 The hinder edge of the first dorsal fin is about at the level of 

 the forty-third ganglion, and that of the second dorsal at the 

 fifty-seventh ganglion. The buds are rather more concentrated. 



In fig. 4 are reconstructed the buds of two dorsal fins of 

 an embryo 19 mm. in length. It is important to notice that 

 there is, at this stage, no gap between the two fins. The 

 first bud passing towards the second dorsal lies immediately 

 behind the last given off towards the first dorsal fin. 



The two dorsal fins of an embryo 24 mm. long are re- 

 constructed in fip-. 2. In the second dorsal the orio-iu of each 



