st, and Rathke later acknowledged that he had
made an error of observation.) The side walls of the skull grow out from
this base and form a fibrous capsule for the brain. The cranial section
of the chorda itself shows no sign of segmentation; but later on the
cranial portion of the chorda-sheath ossifies, like the vertebrae, from
several centres. The vomer, which, in the classical form of the
vertebral theory of the skull, was the centrum of the fourth, or
foremost, cranial vertebra, does not, according to Rathke, develop in
continuity with the cranial basis and the chorda sheath, but develops
separately in the facial region.

Von Baer, like Rathke at this time, was also to some extent a believer
in the vertebral theory of the skull. In his second volume (1834, pub.
1837) he holds that the development of the skull, as the sum of the
anterior vertebral arches, is in general the same as that of the other
neural arches, and is modified only by the great bulk of the brain
(_Entwickelungsgeschichte_, ii., p. 99). He had, however, some doubts as
to the entire correctness of the vertebral theory, doubts suggested by a
study of the developing skull. "In the course of the formation of the
head in the higher animals, something additional is introduced which
does not originally belong to the cranial vertebrae. At first we see the
vertebration in the hinder region of the skull very clearly. Afterwards
it becomes suddenly indistinct, as if some new formation overlaid it"
(i., p. 194).

Even more clearly is his doubt expressed in his paper on _Cyprinus_.
"Upon the formation of the vertebral column only this need be said, that
at this stage the notochord is very clearly seen, and the upper and
lower arches and spinous processes are visible right to the end of the
tail, but the separation into vertebrae ceases abruptly where the back
passes into the head. I do not hesitate to assert _that bony fish, too,
have at this stage an unsegmented cartilaginous cranium_ (as
cartilaginous fish have all their life), the prominences and hollows of
which constitute its only resemblance with the vertebral type" (1835, p.
19).

A convinced supporter of the vertebral theory was Johannes Mueller, who,
in his classical memoir on the Myxinoids,[203] discussed at some length
the relation between the development of the vertebrae and the development
of the skull. His memoir is principally devoted to comparative anatomy,
but in treating of the skeleton he