e development of the ear-ossicles
founded what we may justly call the classical theory of their
homologies. His views were attacked and in some points rectified, but
the main homologies he established are even now accepted by many,
perhaps the majority of morphologists.

In a paper of 1838 on the comparative embryology of the skull in
Amphibia,[208] Reichert added to his results for mammals and birds an
account of the fate of the first and second visceral arches in Anura and
Urodela.

The first visceral arch, he found, gave in Amphibia practically the same
structures as in the higher Vertebrates. Its skeleton segmented, as in
mammals and birds, into three parts; the upper part gave rise to the
palatine and pterygoid in Anura, but seemed to disappear in Urodeles,
where the so-called palatine and pterygoid developed in the mucous
membrane of the mouth; the middle part gave, as in birds, the quadrate,
which formed a suspensorium for both arches; the lower part, as Meckel's
cartilage, formed a foundation for the bones of the lower jaw. Of arch
II., the lower part became the horn of the hyoid, the upper part had a
varying fate. In some Anura it formed the ossicle of the ear (homologue
of the columella of birds and the stapes of mammals), in others it
disappeared. In reptiles the upper segment of the second arch formed, as
in birds, the columella.

The account of the metamorphoses of the visceral arches in Amphibia
forms only a small part of Reichert's memoir of 1838, the chief object
of which was to discover the general "typus" of the vertebrate skull,
and to follow out its modifications in the different classes. Von Baer
had shown that the generalised type appeared most clearly in the early
embryo; Reichert therefore sought the archetype of the skull in the
developing embryo. He brought to his task the preconceived notion that
the skull could be reduced to an assemblage of vertebrae, but he saw that
comparative anatomy alone could not effect this reduction; he had
recourse, therefore, to embryology, hoping to find in the simplified
structure of the embryo clear indications of three primitive cranial
vertebrae (p. 121, 1837).

In the head he distinguished two tubes, the upper formed by the dorsal
plates, the lower by the ventral or visceral plates. Both of these tubes
were derived from the serous or animal layer (_cf._ von Baer, _supra_,
p. 118). The walls of the lower tube were formed by the visceral
processes, within