760 
Proceedings of the Poyal Society 
position, in the embryo the limb grows out in a plane parallel to 
the axis of the body ; and as there is every indication that to the 
Selachians we must usually look for the most primitive condition 
of vertebrate organs, this is a strong argument against Gegenhaur’s 
hypothesis. Based on a study of the development of the limbs in 
Selachians, is the view of the morphology of the limbs advocated 
by Thacker, Mivart, and Balfour, and especially supported by the 
researches of the latter. According to this view, which like the 
branchial hypothesis of Gegenbaur, makes no attempt to bring the 
supposed original condition into relation with the organisation of 
any invertebrate form, the two pairs of limbs in fishes are the speci- 
ally modified remnants of a longitudinal fold, which originally ran 
along each side of the body, and was similar in structure to the 
median folds from which the unpaired fins are derived. The em- 
bryological facts which support this view are briefly thus. In 
Selachians the first rudiments of the paired fins appear as slight 
longitudinal ridge-like thickenings of the epiblast similar to the 
rudiments of the unpaired fins. There are two such ridges on each 
side — one anterior behind the last branchial arch, one posterior on 
the level of the cloaca. In most Elasmobranch embryos, especially 
in Torpedo, they are connected by a line of columnar epiblast cells, 
which very soon disappears. The presence of this connecting ridge 
is suggestive of the original continuity of the two fins, on each side. 
As the fin grows out mesoblast extends into it, and in the mesoblast 
appears embryonic cartilage, which breaks up into a longitudinal 
series of rays, and these are continuous at their base with a 
longitudinal bar termed by Balfour the basipterygium. From the 
anterior end of this longitudinal cartilage there pass off an upward 
and a downward process, which form the rudiment of the limb girdle. 
The longitudinal bar may be due to secondary development, the 
series of cartilaginous rays being the primitive part of the fin which 
is thus reduced to the same structure as an unpaired fin. The 
pelvic fin in a Selachian never departs much from the primitive ar- 
rangement, the only alteration being that the basipterygium segments 
between the anterior and succeeding ray, and the posterior end of 
it segments off as the terminal ray. In the pectoral fin the changes 
are more extensive : the basipterygium is rotated outwards from the 
body until it forms the posterior border of the limb, and constitutes 
