. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 FIG. 122.—Diagrams illustrating theories of origin of appendages
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - col
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1912
