DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 795 
IX. Tae Fins. 
Median Unpaired Fins.—The development of a median epidermal crest (ef, Pl. V. 
fig. 11; Pl. XIIL fig. 3; Pl. 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 Pl. XIX. fig. 5; Pl. XIII. fig. 6; Pl. XVI. 
fig. 1). The extent covered by this fin (ef) varies in different species, thus in the young 
of Trigla gurnardus (Pl. XII. fig. 1) it never extends quite so far forward as in the forms, 
e.g., Gadus eglefinus (Pl. XIV. fig. 1), G. morrhua, G. merlangus (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 (P]. 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 toa 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 larve 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 merlangus (Pl. XVI. fig. 2), Molva vulgaris (Pl. XVIL. 
fig. 9), and Solea vulgaris (Pl. XVIL 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. eglefinus, &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 Plewronides limanda (P). XVI. figs. 3, 6) and Trigla gurnardus 
(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 
