Summing up as regards the cranial vertebrae Rathke writes, "We find
that the four different groups of bones, consisting of the basioccipital
with its intercalary (the supraoccipital), the basisphenoid with its
intercalaries (parietals), the presphenoid with its intercalaries
(frontals), and the ethmoid with its outgrowths (turbinals and
cribriform plate), taking them in order from behind forwards, show an
increasing divergence from the plan according to which vertebrae as
commonly understood develop, so that the basioccipital shows the
greatest resemblance to a vertebra, the ethmoid the least" (p. 30).

In a posthumous volume published in 1861 the same opinion is put
forward. "In the head, too," he writes, "some vertebrae can be
recognised, although in a more or less modified form. Yet at most only
four cranial vertebrae can be assumed, and these differ from ordinary
well-developed vertebrae in their manner of formation the more the
farther forward they lie."[214]

Rathke was an able and careful critic of the vertebral theory of the
skull, but he accepted it in the main. Actual attack on the theory upon
embryological grounds was begun by C. Vogt, in his work on the
development of _Coregonus_,[215] and in his paper on the development of
_Alytes_.[216] He described for _Coregonus_ an origin of the skull in the
main similar to that established by Rathke for the adder. There was a
"nuchal plate" in which the front end of the notochord was imbedded; the
notochord ended at the level of the labyrinth; there were two lateral
bands, comparable to Rathke's lateral trabeculae; a "facial plate" was
also formed, which seems on the whole equivalent to the plate formed by
the fused anterior ends of the trabeculae. A little later the cranium
formed a complete cartilaginous box surrounding the brain, very similar
to the adult cranium of a shark.

In his criticism of the vertebral theory of the skull, Vogt started by
defining the vertebra as a ring formed round the chorda. Now since only
the occipital segment of the skull is formed actually round the
notochord, the parts of the skull lying in front of this cannot
themselves be vertebrae, though they may be considered as prolongations
of the occipital or nuchal vertebra. "We must regard the nuchal plate as
a true vertebra, modified, it is true, in its formation and development
by its particular functions. Now, since the notochord ends with the
nuchal plate we can no longer regard as ve