890 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



in Pleuronectes Jlesus, ^ mcn m length, a space occurs — evidently filled by gelatinous or 

 lymphatic matter with widely separated nuclei. 



Pectoral Fins. — In the larvae of the wolf-fish hatched at the time the eggs were obtained 

 (viz., January) the movements of the pectoral fins (PL XX. fig. 5) were as active as those 

 in the salmon on its escape from the ovum. Their motion within the egg, however, was 

 not noticed at this period. As the season advanced these organs showed more vigorous 

 motions, and were kept in rapid vibration when swimming. On the 1st of April they pre- 

 sented a crenate margin, and before the end of that month attained great development as 

 large fan-shaped expansions, but during the stages under observation their axes remained 

 more or less vertical, as indeed they are in the adult. A. comparison with the salmon is 

 interesting, since, in the wolf-fish, the pectorals remain very large throughout life, whereas 

 in the adult salmon they are of moderate dimensions. Viewed laterally in the newly-hatched 

 salmon (PL XXII. fig. 4) they are very prominent, rising above the line of the dorsum and 

 its median marginal fin. In vertical transverse section their tips far exceed the dorsum, 

 and thus they present a great contrast to those of the wolf -fish, which do not at this stage 

 reach so high. Their plane is nearly parallel with that of the body, and they are more or 

 less rounded in form. On the fifteenth day they show embryonic fin-rays, and distinctly 

 droop to the sides, and do not extend so much above the dorsum (PL XXII. fig. 8). 

 As development proceeds, they gradually increase in size, and by the rotation of their 

 axes they, in the fourth or fifth week, assume a horizontal position (PL XXII. fig. 10). 

 They form, indeed, two shark-like organs which, when the fishes are resting on the stones 

 at the bottom, are often moved in a reptant fashion, after the manner of Chelonian fore- 

 feet. Moreover, the fin of the parr, at a somewhat later stage, becomes less rounded 

 (more lanceolate) than in the earlier form. In horizontal sections of post-larval Gadoids, 

 £-| inch long, the free fin-plate of cartilage is separated by an interval from a proximal 

 (coraco-scapular) plate, internal to which is the hyaline clavicular rod. The interval 

 referred to is filled up by deeply stained cellular tissue which forms a thick protruding 

 band. 



Ventral Fins. — A similar change takes place in the shape of the ventral fins in the 

 salmon, from the rounded form of the seventh day salmon (PL XXVIII. fig. 2) to the lan- 

 ceolate outline of the adult. These organs (ventral fins) are sometimes used for support 

 when resting on the bottom. In the early larva only the embryonic fin-rays are present, but 

 at the end of the third or fourth week there is externally a pale margin, with few cells, which 

 increase from without inward until a mass of cells — traversed by the fin-rays — appears. 



The ventral fins in marine fishes develop late in larval life, one of the most rudi- 

 mentary stages occurring in a post-larval Ammodytes {?), £ inch in length. On the 

 flattened ventral surface two angular knobs appear laterally below the posterior hepatic 

 region. In transverse section the epidermic evagination exhibits an inner columnar layer, 

 with a dense central core, limited internally by the thin peritoneum. A similar appearance 

 is presented by the ventral fin-buds in Gastrosteus spinachia at a late larval stage. In 

 Pleuronectes Jlesus, 2 \ inch in length, the buds are more advanced and have the form of 



