of Eclinhurgh, Session 1883—84. 
759 
3. Critical Note on the latest Theory in Vertebrate 
Morphology. By Mr J. T. Cunningham, B.A. 
In the attempt to trace the vertebrate organisation by comparative 
anatomy and embryology back to a simpler state, the origin of the 
limbs has been made the subject of various hypotheses. Many 
years ago Gegenbaur brought forward a method of regarding the 
morphology of the limbs, by which each could be derived from a gill 
arch supporting a series of gill rays, from such a system as forms the 
skeleton of a gill in a typical Selachian of the present day. This 
comparison was instituted without any particular stress being laid 
on the relation of the ancestral vertebrate to invertebrate forms. 
He supposed that the central ray of the series in the branchia 
gradually grew more prominent, and as it increased in length the 
rays near it lost their attachment to the arch, and became articulated 
to the sides of the central ray : in this way he obtained an imaginary 
limb skeleton which he called the Archipterygium. 
This theory is usually supported by reference to the structure of 
the limb skeleton of Ceratodus. The shoulder girdle of this animal 
is an arch of cartilage more or less ossified, with the upper end of 
which the free limb articulates. This limb is composed of an axial 
rod of cartilage broken up into a series of short segments, and on each 
side of this cartilaginous axis a series of cartilaginous rays is situated : 
each ray articulating with one of the segments of the axial rod. 
In another Dipnoan, Protopteriis, there is a still more striking 
indication of the relation between limb and branchia. In this 
animal the shoulder girdle, as Wiedersheim pointed out, has a 
deeper position than it has in other fishes, a position in relation 
to the surface of the body similar to that of the branchial skele- 
ton. In the second place, the shoulder girdle of this animal bears 
throughout hfe functionally active external giUs j and thirdly, not 
only the shoulder muscles, but the whole limb as well, are innervated 
in great part by branches of the vagus nerve. 
On this hypothesis of the morphology of the limbs, the original 
position of the limb would have been such, that the plane in which 
it was expanded was vertical to the longitudinal axis of the body, 
as it is in Teleosteans and Ganoids. But in Selachians, although in 
adult life the plane of the limb in most cases approaches this 
