NOTES ON MEDIAN AND PAIRED WNS OF FISH. 333 Notes on the Development, Structure, and Origin of the Median and Paired Fins of Fish. By Edwin S. Oootii-icli, F.R.S., Fellow of Merton College, Oxford. With Plat.es 10—14. CONTENTS. PAGE INTRODUCTION ...... 334 The Lateral Fold Theory . .335 The Gill-arch Theory . .340 Objections to the Gill-arch Theory . 342 The Apparent Migration of Fins .... 343 The Faithfulness of Muscle and Nerve . 348 The Shifting of Limbs Explained . .349 OBSERVATIONS ON THE STRUCTURE AND DEVELOPMENT OF THE FINS OF ELASMOBRANCHS ..... 352 Development of the Median Fins of Scyllium canicula . 353 Development of the Skeleton of the Median Fins . 355 Development of the Paired Fins .... 356 The Concordance between the Muscles and the Radials . 357 Concentration ..... 358 The Fusion of the Muscle-buds .... 359 The Position of the Fins . .360 ON THE NERVE SUPPLY OF THE FINS .... 363 The Plexus of the Dorsal Fins . .364 On the Real Nature of the Nerve-plexus . 366 Experiments on the Nerve Supply of the Fins . 369 SUMMARY AND CONCLUSION ..... 371 VOL. 50, PART 2.—NEW SERIES. 24 334 EDWIN S. GOODRICH. INTRODUCTION. A GREAT deal has been written in the last few years about the structure, development, and origin of the paired fins of fish, yet two rival and incompatible theories are still preva- lent. According to the theory put forth by Gegenbaur (14, 15,16), the paired fins have been derived from gill structures, the gill-arch having been modified into the limb-girdle, and the fin itself, with its skeleton, having been derived from the gill-flap or septum, and its supporting gill-rays. This may shortly be called the "gill-arch theory." The second theory, that of Balfour (1, 2), Thacher (35), and Mivart (23), holds that the paired fins are of the same nature as the unpaired median fins. According to this view, the limbs have been derived from paired longitudinal fin-folds, in which skeletal supports, the radials, or somactidia (Lankester), became developed as in the median fins, and subsequently gave rise to the limb-girdles. This may be called the "lateral fin-fold theory." Each of these theories may claim to have among its numerous supporters the names of some of the most eminent exponents of the morphology of the vertebrates. Dohrn (10), Hasvvell (20), Eabl (31), Mollier (24, 25,, 26), Harrison (19), Wiedersheim (36), A. Smith Woodward (37), and Dean (9) have written in favour of the lateral fold theory; Davidoff (8), Fiirbringer (12), Braus (3-7), and others have supported its rival. It is unnecessary for me in these notes to give a history of the discussions to which the question has given rise; the literature has been recently reviewed by Mollier and Braus, and the whole subject is familiar to zoologists. But there are certain essential points which seem to be in danger of being obscured from view in a cloud of controversy, and it is in the hope of clearing up some of these points and of filling up certain gaps in the evidence that these notes have been published. As I am anxious to keep this paper within reasonable limits and not to overburden the already very bulky literature on the subject of the origin of the paired limbs, only some aspects NOTES ON MEDIAN AND I'ATBKD FINS OP FISH. 335 of the problem will be dealt with in detail. A. brief and some- what dogmatic statement of the case is made at the beginning, followed by a description of my own observations, and ending with a short summary. THE LATERAL FOLD THEOUY. Balfour's conception of an originally continuous fin-fold, reaching from the pectoral to the pelvic region (1) is dis- credited because it has been found only (as an epidermal fold) in those forms, like Torpedo, in which the pectoral fins reach the pelvic fins in the adult, a condition which is probably rightly considered as secondai'y. Moreover, the appearance of an epidermal longitudinal fold, as a first indication of the development of the paired fins, is considered to be of little importance, and its presence between the paired fins is denied in sharks (Mollier 24, Braus 4, etc.). Now, the continuity of the pectoral with the pelvic fin- fold is not an essential point. The important thing is to recognise that the paired fins always arise as a longi- tudinal fold or ridge, similar to that which gives rise to the median fins. That this is really the case is now admitted by all (Braus 7). Even in Ceratodus, where the paired fins in the adult are set at a pronounced angle to the long axis of the body, they make their first appearance as longitudinal ridges (Semon 33). • Possibly from the very first, in phylogeny, the paired fins were discontinuous, and differentiated into pectoral and pelvic. For conclusive evidence on this point we must look to palaeontology; and it has not yet been obtained. But there is some evidence to be gathered from comparative anatomy and embryology in favour of Balfour's view, as has frequently been pointed out (Dohrn 10, Mollier 24). For instance, the musculature of the fins is developed in Elasmobranchs, from buds given off from the ventral ends of the myotomes, and these buds have been shown to be pro- duced not only on the myotomes in the region of the fins, but 336 EDWIN S. GOODRICH. on all the trunk myotomes situated between the pectoral and the pelvic fins in such sharks as Pristiurus and Scyllium, in which these fins are widely separated in the adult (Dohrn 10, Rabl 31, Braus 4, and p. 343 below). Many of the inter- mediate buds seem to disappear entirely during development. In those segments which are near the fins the buds become better developed and more persistent, and a large number pass into the fin-fold. Muscle buds are also found in front of the pectoral fin and behind the pelvic fin, dwindling in size as they are farther removed from the fin-base. Thus, in these sharks, the muscles of the paired fins are formed by the great development in two regions of a continuous series of muscle buds, vanishing posteriorly behind the cloaca. The manner in which these vestigial buds disappear by reduction at either end of the fin rudiment, and in which the persistent buds become concentrated at the relatively narrowing base of the fin, has been admirably described and figured by Mollier (24) and Braus. The fin-base of the adult occupies much less space relatively than the fin-fold of the embryo. Now, the radial fin-muscles being developed from buds of the myotomes, naturally receive their motor nerve-supply from the ventral roots of the spinal nerves, and these corre- spond in number to the myotomes which share in the forma- tion of the musculature. Owing to concentration, the nerves are found to converge toward the base of the fin. In front and behind the nerves may be drawn together so as to form a "collector" nerve or compound stem. It is part of the lateral fold theory to suppose that the endoskeleton of the paired fins has been derived from a series of cartilaginous rods, radials, or somactidia, similar to those of the median fins (Thacher 35, Mivart 23). The various types of fin-skeleton, with their cartilage rays and basal pieces, would have been developed from such originally segmental radials by concentration and fusion. To this contention it is objected that in development the radials of Elasniobranchs arise in a continuons rudiment—a plate of procartilaginous NOTES ON MEDIAN AND PAlllliD FINS Oi' PISH. 337 mesenchyme (Balfonr 2, Mollier 24, Ruge 32; and p. 357 below). It may be answered (Dohrn 10, Mollier 24) that, the radials being closely approximated, their procartilaginous rudiments 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 number 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 by 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 prochondral 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 early 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, in 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).