Gk General Notes. [January, 
cercy the diphycercal continuity of the fin-series has remained 
practically unimpaired, thus affording the necessary proof of the 
serial homology of the entire series of median fin-rays and their 
intermediary supports. (Previous authors failing to attack this 
part of the problem by the light of the ontogeny of a diphycer- 
cal eel-like type have missed the solution of one of the most im- 
portant minor parts of a rational theory of the median fins, since 
it is otherwise impossible to prove such a homology in forms 
with atrophied intervals between the vertical fins.) The meso- 
blastic ‘skeletogenous tract from which the median fin-rays. and 
their supports are developed, is continuous in the median line of 
the urosome, above, below and almost over the end of the chorda 
in fish embryos; such a continuity affords an explanation of why 
the median fin-rays form an uninterrupted series in cases of per- 
fect diphycercy (Fig. 4), or where the archaic has not been re- 
placed by a specialized mode of development, in the course of 
which discontinuity has arisen (Protopterus). 
4. Heterocercy—Heterocercy affects only the end of the chor- 
dal axis, which is bent upwards, and as a result of this it and the 
subsequently formed terminal vertebral segments are consolidated 
into a urostyle (many Teleostei), above and below which epaxial 
and hypaxial skeletal elements are formed, of which the former 
are, however, often aborted, and the latter widened as supports 
for the caudal system of rays. 
is condition appears to result from two causes: (1) Great 
activity of growth in the terminal hypaxial part of the primitive 
caudal fin-fold in consequence of which the chorda is shoved up- 
wards ; and (2) from the actions of the animal in using the result- 
ing expanded, hypaxial, caudal, ray-bearing fold in swimming ; 
the strokes of the fin in action, owing to the resistance offered by 
the water, tend to throw up the somatic axis, just as an oar tends 
to be thrown upward in sculling. 
Since the hypaxial fold may be developed at some distance 
from the end of the tail, in the more specialized forms (Lepidos- 
teus, Fig.6; Gasterosteus) a more or less extensively free portion 
aS, 
2RR 
ot the lophocercal caudal axis is left to project (Fig. 7) during 
the growth of the true or secondary caudal, the rays of which 
are mostly hypaxial and serially homologous with those of the 
anal. The exserted part of the larval axis alluded to above, may 
be called the ofzsthure, in reference to its position in relation to 
` 
