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of Edinburgh, Session 1883 - 84 . 
that part of the adult fin called metapterygium by Gegenbaur, and 
then becomes segmented off from the pectoral girdle articulating 
with its hinder edge. The propterygium and mesopterygium are 
merely the anterior part of the original basipterygium which divides 
into two pieces, articulating directly with the pectoral girdle. Thus 
the metapterygium is the homologue of the basal cartilage of the 
pelvic fin. It is obvious that the facts of the development dis- 
covered by Balfour are absolutely incompatible with Gegenbaur’s 
view of the origin of the limb from a branchial arch and its rays. 
According to Gegenbaur’s view, the limb of Ceratodus is the most 
primitive form, and the metapterygium in the Elasmobranch 
corresponds to the central axis in Ceratodus, the posterior rays 
having disappeared. The development of the fins in the dog fish 
shows that the metapterygium is the basal part of the fin grown 
outwards, and could never have borne posterior rays. 
In the earliest sketch of the genealogy of vertebrates put forward 
by Dr Dohrn in 1875,* he also maintained that the vertebrate 
paired limbs originated from gills, not gills constructed on the plan 
of those of a fish of the present day, but branched tree-like giUs 
resembling those of living annelids, such, for example, as those of 
Arenicola piscatorum. He supposed that special muscles first became 
separated from the dermo-muscular tube, in order to move the gills for 
the sake of more efficient respiration that then the gills began to be 
used as locomotive organs ; and finally, when respiration was confined 
to the anterior giU slits, two pair of these gills were preserved, and com- 
pletely changed into locomotive organs on account of the suitability of 
their position for keeping the equilibrium of the cylindrical body in 
the water. The assumption in this view, that a limb was originally 
a process of a single somite of the body, could not well be main- 
tained in face of the facts of the structure and development of the 
fins of fishes. In Elasmobranchs it is obvious that a lateral fin be- 
longs to several somites of the body, not to a single one. Balfour 
found that the muscles of a limb in an embryo Elasmobranch were 
derived from buds of several muscle plates, that is from several 
somites. In a recent paper f on the orgin of the paired and unpaired 
* Ursprung der WirbeltMere, Leipzig, 1875. 
+ “Studien zur Urgeschichte des Wirbelthierkorpers VI.” Mittheil. aus der 
Zoologisdien Station zu Neapel, Band V. Heft 1. 
