anal. They had no direct
connection with the vertebral column, and seemed therefore to belong to
what Carus[201] had called the visceral or splanchno-skeleton. He did not,
however, let this distinction hinder him from asserting the substantial
homology of all the gill-arches _inter se_, the first two included.

Rathke's discoveries relative to the development of the jaws, the hyoid
and the operculum, enabled him to make short work of the homologies
proposed for them by the transcendentalists. He could prove from
embryology that the jaws were not the equivalent of limbs, as so many
Okenians believed. He could reject, with a mere reference to the facts
of development, Geoffroy's comparison of the hyoid and the
branchiostegal rays in fish with sternum and ribs. He could show the
emptiness of the attempts made by Carus, Treviranus, de Blainville and
Geoffroy, to establish by anatomical comparison the homologies of the
opercular bones, for he could show that these bones were peculiar to
fish, and were scarcely indicated, and that only temporarily, in the
development of other Vertebrates.[202] He did not, however, himself
realise the relation of the ear-ossicles to the gill-arches, though he
knew that Spix and Geoffroy were quite wrong in homologising them with
the opercular bones in fish. He described, it is true, the development
of the external meatus of the ear and the Eustachian tube from the slit
which appears between the first and the second arch, as Huschke had done
before him; he described, in confirmation of Meckel, the "Meckelian
process" of the hammer running down inside the lower jaw; but the
discovery of the true homologies of the ear-ossicles was not made until
a year or two later by Reichert.

In his further study of the development of _Blennius viviparus_, Rathke
observed some important facts about the development of the vertebral
column and skull. He found that the vertebral centra were first formed
as rings in the chorda-sheath, which give off neural and haemal
processes. The vertebra later ossifies from four centres. The chorda
(notochord) is prolonged some little way into the head, and the base of
the cranium is formed by the expanded sheath, which reaches forward in
front of the end of the notochord. This cranial basis shows a division
into three segments, in which Rathke was inclined to see an indication
of three cranial vertebrae. (It turned out that this division into three
segments did not really exi