ne
(_urohyal_) which is attached in front by two strong tendons to the
horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had
seen in this bone the homologue of the sternum. Geoffroy adopts this
view, but considers that this bone alone cannot represent the whole
sternum. He finds the representatives of other bones of the sternum in
the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the
_ceratohyal_) which are comprised in the hyoid arch. But he is
immediately met by the difficulty that this complex of bones is
situated in front of the pectoral girdle, whereas the sternum in
higher Vertebrates lies behind the pectoral girdle. He reflects,
however, that the gills of fish, situated in front of the clavicles,
are merely the lungs under another name. The gills have become shifted
forward by a metastasis similar to that which brought the whole
thoracic organs far forward in fish. This being so, their supporting
elements, the sternum and the ribs, must have moved with them, and are
hence to be found in front of the pectoral girdle.

[Illustration: FIG. 1.--Hyoid Arch of the Conger. (Original.)]

Geoffroy's next step is to point out that the only possible homologues
of sternal ribs are the branchiostegal rays, which arise from the large
bones of the hyoid arch. If these are sternal ribs, the bones to which
they are attached must be the hyo- and hyposternals or "annexes," the
bones from which in birds the ribs take their origin.

The unpaired sternal bone (_urohyal_) cannot be homologous with the
entosternal, for it has no connections with the annexes. He decides that
it must represent the episternals, for in some young birds there is a
two-headed episternal to which two strong tendons are attached, just in
the same way as the unpaired piece in fish is bound to the bones of the
hyoid by two tendons. "Thus it is not the sternum as a whole that has
shifted in front of the clavicles and covered with its side pieces the
gills placed there; it is a piece exclusively piscine, in the sense that
it is only in the class of fishes that it reaches the _maximum_ of its
development" (p. 83).

To sum up, the sternum in all four vertebrate classes is composed of the
same elements, arranged always in the same way. "One is ... led to the
conception of an ideal type of sternum for all Vertebrates, which then,
considered from a lower standpoint, resolves itself into several
secondary forms according as the whole or