-slit, or, as Rathke here prefers to call it, pharyngeal
slit, closes completely in snakes and in Urodeles. It forms the
Eustachian tube in all other Tetrapoda. As regards the vertebrae, Rathke
describes them as being formed in the sheath of the chorda from paired
rudiments, each of which sends two branches upwards, and two branches
downwards. The two inner pairs of processes coalesce round the chorda,
and later form the centrum; the upper outer pair meet above the spinal
column; the lower outer pair form ribs. The odontoid process of the axis
vertebra is the centrum of the atlas (p. 120). The formation of
vertebral rudiments begins close behind the ear-labyrinth, but in front
of this the chorda-sheath gives origin to a flat membranous plate which
afterwards becomes cartilaginous. This plate reaches forward below the
third cerebral vesicle as far as the infundibulum. The notochord ends in
this plate, which is the _basis cranii_, just at the level of the
ear-labyrinth. In no Vertebrate does the notochord extend farther
forward (p. 122). The _basis cranii_ gives off three trabeculae. The
middle one is small and sticks up behind the infundibulum; it is absent
in fish and Amphibia, and soon disappears during the development of the
higher forms. The lateral trabeculae are long bars which curve round the
infundibulum and reach nearly to the front end of the head. Together
they are lyre-shaped. The cranial basis and the trabeculae are formed,
like the vertebrae, in the sheath of the notochord, and the only
differences between the two in the early stage of their development are
that the formative mass for the cranial basis is much greater in amount
than that for the vertebrae, and that the cranial basis by means of its
processes, the trabeculae, reaches well in front of the terminal portion
of the notochord (p. 36). The capsule for the ear-labyrinth develops
quite independently of the cranial basis and the notochord. It resembles
on its first appearance, in form, position, composition, and
connections, the ear-capsule of Cyclostomes, and so do the ear-capsules
of all embryonic Vertebrates (p. 39). It manifests clearly the embryonic
archetype, ... "there exists one single and original plan of formation,
as we may suppose, upon which is built the labyrinth of Vertebrates in
general" (p. 40). When ossification sets in, the ear-capsule forms three
bones, of which two fuse with the supraoccipital and exoccipitals.

[Illustration: FIG