II4 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
tilage persists for some time, or through life, but elsewhere it disap- 
pears and the elements unite by symphysis. The same groups of non- 
placental mammals are characterized by the presence of marsupial 
bones (fig. 121). These are preformed in cartilage and extend for- 
ward from either pubis in the ventral abdominal wall. Their homol- 
ogy is very uncertain; but they are not the ypsiloid of the urodeles. 
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_ Fic. 122.—Diagrams illustrating theories of origin of appendages. 4A, B, C, origin of 
biserial appendage (C) from gill arch (A); D, biserial appendage (archipterygium); Z, F, 
evolution of elasmobranch fin; G, dotted lines indicate parts involved in origin of leg from 
fin; H, dotted parts show another view of origin of elements of leg. 
The Free Appendages. 
These are of two kinds, the paired fins (ichthyopterygia) of the 
fishes and the legs or their modifications (chiropterygia) found in all 
classes of tetrapoda. The formér is merely a mechanism for altering 
the position of the body in the water, and requires a small amount of 
flexibility, being moved as a whole. The assumption of terestrial. 
habits necessitates the support of the body above the ground and its 
propulsion. Hence the chiropterygium must have a firmer skeleton, 
with at the same time joints for motion and intrinsic muscles to move the 
parts on each other. The chiropterygium was undoubtedly derived 
from the fish fin, but the problem of how the change was made has not 
been solved. Only paleontology can give the answer. 
There are two views as to the origin of the chiropterygium, both based upon the 
loss of certain ‘parts and the persistence of others in a modified form. One view 
assumes the persistence of a basal as the framework (humerus or radius) of the 
