DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 795 



IX. The Fins. 



Median Unpaired Fins. — The development of a median epidermal crest (ef, PI. V. 

 fig. 11 ; PI. XIII. fig. 3 ; PI. XIX. fig. 10), extending along the median dorsal line from 

 the cephalic region round the end of the tail, and along a portion of the under surface of 

 the caudal trunk, is an early and noticeable feature in the embryos of Teleostean fishes, 

 with probably few exceptions (e.g., Hippocampus). Soon after the tail is detached from 

 the yolk-surface, within a day or two after the closure of the blastopore, a minute fold of 

 epiblast projects as a ridge along the whole course just indicated. It grows in vertical 

 breadth, being pushed out in the form of an epiblastic fold, and shortly before the extrusion 

 of the embryo is quite a broad membrane, especially well developed in the hind trunk and 

 caudal region. On account of its superficial extent — while the embryo is within the egg 

 — it is creased and much folded about the body ; but on the embryo issuing from the 

 ovum the membrane rapidly straightens out and becomes erect. It apparently continues 

 to grow after extrusion, a newly hatched embryo having a much less extensive median 

 membrane than one a few days old (compare PI. XIX. fig. 5 ; PL XIII. fig. 6 ; PI. XVI. 

 fig. 1). The extent covered by this fin (ef) varies in different species, thus in the young 

 of Trigla gumardus (PL XII. fig. 1) it never extends quite so far forward as in the forms, 

 e.g., Gadus ceglefinus (PL XIV. fig. 1), G. morrhua, G. rnerlangus (PL XVI. fig. 2), and 

 Motella (PL XVII. fig. 2) ; its wider portion in fact reaching only to the otocystic region, 

 in front of which its height gradually diminishes, and the fin disappears above the 

 occipital region (PL XVI. fig. 8). In such examples as the Gadoids just mentioned, it is 

 broad and prominent as far forward as the mid-brain, in which region it gradually 

 slopes to a mere ridge. The thinness and transparency of this structure is remarkable. 

 It is so delicate that as the fish progresses through the water it is flexed and waved 

 about with every movement, and on removal from the water the fin collapses at once, 

 and lies like a film on the body. Slight contact with a hard substance immediately 

 injures it, and while in healthy larvae it stands out erect and even, and is per- 

 fectly translucent, it appears crumpled and in many parts opaque when the fish is 

 in a sickly or dying condition, ultimately dissipating or breaking up into needle-like 

 fragments. 



In certain forms, e.g., Gadus rnerlangus (PL XVI. fig. 2), Molva vulgaris (PL XVII. 

 fig. 9), and Solea vulgaris (PL XVII. fig. 13). the pigment, which extends not only over 

 the body, but over the yolk-sac, appears also upon the embryonic fin (ef) ; whereas in 

 Gadus morrhua, G. ceglefinus, &c, no such pigment-corpuscles occur save on the trunk 

 of the fish — the yolk-sac as well as the membrane being destitute of them. It was men- 

 tioned previously that in Pleuronides limanda (PL XVI. figs. 3, 6) and Trigla gumardus 

 (PL XVI. fig. 8) the fin shows during the later larval stages remarkable coloration 

 — in the former species crescentic particles of yellow pigment appearing in regular series 

 along the membrane above and below the caudal trunk during the second week after 



