1885.] Embryology. 93 
the primitive fold, which may therefore be considered the matrix 
of the permanent fins 
In the formation of rays, their supports and musculature, there 
is clearly a close correspondence between the number of ray- 
bearing somites of the body and the one, two or three rays and 
supports which are developed to each segment, and this is mani- 
fested even when heterocercy and its accompanying degenerative 
processes manifest themselves in the caudal region of the most 
specialized forms. 
3. Diphycercy—The most archaic distribution of the median 
fin-rays is a continuous one, (as in Fig. 4), and is hypaxial from 
the vent to the end of the tail and then 
forward dorsally or epaxially ; (Ccelacan- 
thi, Placodermi, Dipnoi, Pleuracanthus). 
Another archaic trait is the perfectly “ 
straight chorda or vertebral axis which 7S 
extends without upward curvature in 
` typically diphycercal forms to the end of 
the urosome. (An archaic trait which 
also marks a phase of the ontogeny of the Teleosts is the Ceela- 
canthous—hollow—condition of the bony portion of the spines 
and their supports.) Fishes with a long eel-like body have 
tended to remain diphycercal, while those whose bodies have 
been abbreviated have tended, with the exception of such forms 
as the Heterosomata, to develop discontinuous median fins which 
have very probably been derived in the first instance, from hyper- 
trophied portions of a continuous series. This hypertrophy in 
some cases involved the whole series, e. g., Platax. The prime- 
val pre-diphycercal or lophocercal condition is mediately followed 
by the next stage (Fig. 5) which, as we have seen, must have 
been developed from a more archaic condi- 
tion or one of true diphycercy. There 
therefore occurs a more or less extensive 
elision or failure to develop a continuous 
i 
of median fins. Embryonic mene rage — Mhs 
therefore fails to exactly recapitulate the 
phases of evolution of the median fins. Even the embryonic rays 
which are of mesoblastic origin do not always form a continuous 
series. They are far more numerous than the permanent rays, 
and are characteristic of the diphycercal condition and rep 
a stage of fin development which may be called the protopterygian 
These views are fully substantiated by the development of the 
caudal skeleton of the eel, in which in spite of its slight hetero- 
1 Another article in the succeeding number will deal with the origin of the fin- 
rays. 
