NOTES ON 3IEDIAN AND I'AlRliD EINS OF FISH. So7 



nieseiicliyme (Balfour 2, Mollier 24, Ruge 32, and p. 357 below). 

 It may be answered (Dolini 10, Mollier 24) that, tlie radials 

 being closely approximated, their procartilaginous rndiments 

 with indefinite borders necessarily merge together to a con- 

 siderable extent. As a matter of fact, the cartilage pieces 

 appear as islands in the vaguely-defined rudiment, which 

 correspond closely in position and nuuiber with the separate 

 elements of the adult fin-skeleton. Some slight indications 

 of recapitulation, some fusion of neighbouring radials, may be 

 detected, which bears out the views so convincingly advocated 

 b}^ Thacher and Mivart. But it cannot be claimed that re- 

 capitulation is complete in this respect in the development of 

 the paired fins. It is obvious, however, that if its absence is 

 considered as evidence against the lateral fold theory it tells 

 with equal force against the gill-arch theory, since 

 ■ the skeleton is, according to this view, also derived from 

 originally separate (branchial) rays. 



But the whole argument against the lateral fold theory 

 collapses when we find that, as Balfour long ago showed, 

 the radials of the median fins likewise arise in a 

 continuous prechondral plate, in the median fins of 

 Elasmobranchs, even when they are separate in the adult 

 (p. 355 below). These median fins are much concentrated, 

 and nothing proves so clearly that the eaily continuity of the 

 rudiments is due to their approximation, for here the original 

 metameric nature of the radials will not be denied. The 

 most enthusiastic supporter of the gill-arch theory would 

 not suppose that the continuous plate represents an early 

 stage in the phylogenetic history of the skeleton of median 

 fins ! Unfortunately, we know but little concerning the 

 development of the skeleton in unconcentrated median fins. 

 Doubtless, in such cases the radials arise separately; Harrison, 

 indeed, has shown this in his valuable paper on the salmon (19) . 



Yet other objections have been brought forward by Braus, 

 iu the elaborate and beautiful memoirs which have of late 

 contributed so much to our knowledge of the structure and 

 development of fins (3, 4, 6, 7). It has been shown that two 



