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Raymond C. Osburn 1 
THE INFERIOR CAUDAL FIN. 
It is important to observe that the inferior caudal fin of Cestracion in 
its earlier stages agrees exactly with the other unpaired fins. This par- 
ticular fin has always been a point of contention among students of the 
limb problem, since in the development of the definitive skeleton the 
rays in most cases arise in direct connection with the hemal spines of 
the axial skeleton, while in no other fin of any fish are the rays known 
to arise in this connection (with the one exception of the superior or 
epichordal portion of the diphycereal caudal fin of Lepidosiren 
(Braus, 04a, o4b), the most highly specialized of the Dipnoi). ‘There 
can be but three views in regard to this difference: 1st, The inferior 
caudal may be considered to have had an entirely different mode of 
origin from the other fins; 2d, it may be looked upon as the type of all 
the unpaired fins under the supposition that the others have lost their 
connection with the axial skeleton; or, 3d, that the connection of the 
rays of this fin with the axial skeleton has come about secondarily. 
Balfour and Parker, pioneers in vertebrate embryology adopted the first 
view, but no one since their time has suggested it. The second view 
has been adopted by the adherents of the gill-arch theory, following 
Gegenbaur’s assertion that the cartilages of the dorsals are only modified 
spinous processes, “ losgeléste und selbstindige1 gewordene Dornfort- 
sitze.” Professor Braus attempts (04a) to support this hypothesis by 
the observation on the superior caudal fin of Lepidosiren already alluded 
io, making this dorsal connection an intermediate step between the in- 
ferior caudal and the dorsals. This sort of argument seems to us hardly 
warranted when the high specialization of Lepidosiren and the wide 
phylogenetic gap separating the modern Dipnoi from the Selachii are 
considered. The third view, viz., that the unpaired fins have all had 
a similar, external origin, and that the inferior caudal has become second- 
arily attached to the axial skeleton for mechanical reasons, is the one 
adopted by the fin-fold theorists. This view was advanced by Dohrn. 
84, and strongly supported by Paul Mayer, 86, who found in the anterior 
part of the inferior caudal fin the evidence of a separate origin of the 
rays of this fin,—‘ ferner wire vom Knorpel noch zu bemerken, dass 
er wie bei allen anderen Flossen erst secundaér mit der Wirbelsaule in 
Verbindung tritt.” Now, Cestracion, at an early stage in the develop- 
ment of the inferior caudal, offers a very striking confirmation of the 
view that this fin has arisen in an external manner similar to the other 
unpaired fins. Fig. 3 shows a section through the fin at a time when the 
mesenchyme plate from which the fin rays develop is entirely separated 
