114 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. 



FIG. 122. Diagrams illustrating theories of origin of appendages. A, B, C, origin of 

 biserial appendage (C) from gill arch (A); D, biserial appendage (archipterygium) ; E, 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 former 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 



