. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 148 MORPHOLOGY OF THE ORGANS OF tion as to how far the sternum of the amphibia is homologous with the similarly named structure in the amniotes. In the amphibia the sternum arises as a pair of longitudinal cartilaginous rods in the connectiAe tissue on the ventral sur- face of the bod)-. These rods soon unite, and form an unpaired plate in the median line between the origin of the fore limbs. In the uro- deles the sternum remains as a small plate just behind the ventral portion of the shoulder girdle, but in th
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 148 MORPHOLOGY OF THE ORGANS OF tion as to how far the sternum of the amphibia is homologous with the similarly named structure in the amniotes. In the amphibia the sternum arises as a pair of longitudinal cartilaginous rods in the connectiAe tissue on the ventral sur- face of the bod)-. These rods soon unite, and form an unpaired plate in the median line between the origin of the fore limbs. In the uro- deles the sternum remains as a small plate just behind the ventral portion of the shoulder girdle, but in the anura it extends farther for- ward. Its median portion is caught between the epicora- coids, and is reduced to a ver)- slender thread ; but in front of the girdle it expands again in a plate, the so-called omostemum. In the uro- deles the sternum is cartilaginous ; but in the anura portions of the omosternum, as well as of the posterior portion (termed xiphisternum, a term adopted from human anatomy), become ossified. The sternum is lacking in the footless amphibia. In the amniotes the sternum arises from the ventral ends of the ribs. The distal ends of these become separated from the rest, and unite to form a pair of ventral rods, which then unite to form the unpaired structure, which in many forms shows evidences of its origin from a series of elements, — stemebrae. The sternum is lacking in snakes and turtles. In the dinosaur Amphiccelias, it is said to have been paired in the adult, the two halves possibly having been united by cartilage. In the lizards it is usually a broad rhomboidal plate. In the birds but few (at most eight) ribs contribute to the sternum, which is a broad plate, and in the ordinary birds bears a strong keel or carina upon its ventral surface. In the flightless birds the keel is absent, and the presence or absence of keel was formerly em- FiG. 156. Sternum and ventral portion of the shoulder girdle of Rana, after W'ieders- heim. cl, claflcle; co, co
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