The median fins are to be regarded as the more primitive. The
fundamental structure of the vertebrate, with its median skeletal axis
and its great muscular mass divided into segments along each side of the
body, indicates that its primitive method of movement was by waves of
lateral flexure, as seen in an Amphioxus, a cyclostome or an eel. The
system of median fins consists in the first instance of a continuous
fin-fold extending round the posterior end of the body--as persists even
in the adult in the existing Dipneusti. A continuous median fin-fold
occurs also in various Teleosts (many deep-sea Teleosts, eels, &c.),
though the highly specialized features in other respects make it
probable that we have here to do with a secondary return to a condition
like the primitive one. In the process of segmentation of the originally
continuous fin-fold we notice first of all a separation of and an
increase in size of that portion of the fin which from its position at
the tip of the tail region is in the most advantageous position for
producing movements of the body. There is thus formed the _caudal_ fin.
In this region there is a greatly increased size of the fin-fold--both
dorsally and ventrally. There is further developed a highly
characteristic asymmetry. In the original symmetrical or _protocercal_ (
= _diphycercal_) type of tail (as seen in a cyclostome, a Dipnoan and in
most fish embryos) the skeletal axis of the body runs straight out to
its tip--the tail fold being equally developed above and below the axis.
In the highly developed caudal fin of the majority of fishes, however,
the fin-fold is developed to a much greater extent on the ventral side,
and correlated with this the skeletal axis is turned upwards as in the
_heterocercal_ tail of sharks and sturgeons. The highest stage in this
evolution of the caudal fin is seen in the Teleostean fishes, where the
ventral tail-fold becomes developed to such an extent as to produce a
secondarily symmetrical appearance (_homocercal_ tail, fig. 4).
[Illustration: From _"Challenger" Reports Zool._, published by H.M.
Stationery Office.