Biometric Evidence in the Problem of the Paired Limbs of the Vertebrates
Year: 1902
Type: article
Abstract: The foregoing characters yield what seems to me convincing evidence that the pectoral fin is subject to changes with respect to the gravity center, i.e., physical changes, which cause it to become more highly specialized than the ventral fin, and that the latter is conservative, after the fashion of the unpaired fins. The result of biometric data, it will be seen, confirms strikingly the views of the fin-fold theorists, most clearly expressed in this particular regard by Wiedersheim in his "Gliedmassenskelet." On the other hand, the present study yields no evidence that there has ever been a migration of the fins in the Gegenbaurian sense; thus, we find no reason to believe that the ventral fin is a structure which has shifted its position from in front hindward; it is indeed in about the same position fore and aft in the adult as in early embryos (20 mm.). It is important, furthermore, that the only fin (leaving anal and caudal out of discussion) whose anterior margin passes forward is the pectoral, since with this condition is correlated the fact that this rim alone is in advance of the gravity center. Thus, as this center passes backward, the posterior margin of the ventral fin is found to follow it pari passu, and, as effecting a counter balance, like equal weights placed in opposite scale pans, the anterior rim of the pectoral passes headward. Again, contrasting the embryos shown drawn to scale in Fig. 1, and referring now to the position of the external gill openings, we note that there is a marked tendency for these openings to pass backward. In the earlier specimens all are widely interspaced and the last is well in front of the fore rim of the pectoral; in the next embryo the openings are drawn together, altogether somewhat tailward; and in the latest, as far at least as to the second opening, the series have taken a position hindward of the rim of the pectoral. In other words, as far as these openings are concerned, and with them the outerpart at least of the gill canal, it is evident that in the older stages a definite migration has taken place, to a degree that, in terms of the total length of the animal, the gill openings come to occupy a position in the body wall which is largely behind that of the earlier stages. Numerically considered, the interval between gill openings I and V is about 8 per cent of total length in stages 1 and 3; but in the latter the region of the gill openings is nearly 5 per cent farther tailward; therefore, in this stage the openings occupy a position in the body surface which is about five-eighths new, i.e., never before occupied by gills. From this it follows that the gill region, at least in its outer part, shows no affinity during proportional growth with the neighboring region of the pectoral fin. In fact, from an early stage onward, they are evidently growing in opposite directions. If, now, we figure the matter up, we find that the distance the gill openings pass tailward (contrasting embryos varying from about 20 to 300 mm.), added to the distance which the anterior rim of the pectoral passes headward, makes the considerable sum of 20 per cent of the length of the animal. And from this it follows that there occurs in this region a process of "shortening up," which is surely enough to account for the presence of anastomosing vessels, blended muscle bands, plexus of nerves, and "collectors." The presence of such complicated structures, especially in the region of the anterior or posterior rim of a (pectoral) fin, is therefore to no little degree a product of the mechanical needs of fin migration.
Source: The American Naturalist
DOI: 10.1086/278225
Author: Bashford Dean
Language: English
Subfield: Aquatic Science
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1991006328