on, one behind the other, in the basilar
cartilage."[212]

Rathke was very cautious about accepting the vertebral theory of the
skull; he saw that the facts of development were not altogether
favourable to the theory, and he gave his adherence with many
reservations and saving clauses. His general attitude may be summed up
as follows.[213]

The chorda sheath is the common matrix of the vertebrae and of a large
part of the skull. The basilar plate and the trabeculae, which are
developed from the chorda sheath, give origin to three bones, which
might possibly be considered equivalent to vertebral centra--the
basioccipital, the basisphenoid, and the _Riechbein_ (ethmoid). The
_Riechbein_ develops from the fused ends of the trabeculae. The
presphenoid might also be considered as a vertebral body, but it
develops independently of the basilar plate and trabeculae.

Now of these bones, the basioccipital is in every way equivalent to a
vertebral centrum, for it develops in the basilar plate round the
notochord. With the exoccipitals, which arise just like neural arches,
it forms a true vertebra. The supraoccipital is an accessory bone
developed in relation to bigger brains. The basisphenoid appears in the
basilar plate, but in front of the notochord, nor does it arise in
exactly the same way as the centrum of a vertebra. The basisphenoid with
the alisphenoids, which develop independently in the side walls of the
brain, may, however, still be considered as forming a vertebra, though
the resemblance is not so great as in the case of the occipital ring.
The presphenoid, being long and pointed, is very unlike a vertebral
body. The orbitosphenoids develop separately from it. The ethmoid also
differs from a vertebra, for it surrounds not the whole nervous axis as
the two hinder "vertebrae" do, but only two prolongations of it, the
olfactory lobes. In its development and final form it shows no
particular resemblance to a vertebra. Its body, the _pars
perpendicularis_ (mesethmoid) shows no similarity with a vertebral
centrum. Completing the three hinder cranial "vertebrae" and roofing in
the brain are the supraoccipital, the parietals and the frontals. The
premaxillaries, vomer, and nasals do not belong to the cranial scheme;
they are covering bones connected with the ethmoid. So, too, the
ear-capsule is not part of the cranial vertebrae, but is rather to be
compared to the intercalary bones in the vertebral column of certain
fish.