| | |
|Second vertebra |Basisphenoid |Alisphenoids |Parietals |
| | | | |
|Third vertebra |Basioccipital |Exoccipitals |Supraoccipital |
+-------------------------------------------------------------------+

The other bones of the skull are not included in the vertebrae, and this
is in large part due to the fact that the sense capsules are formed
separately from the cranium (p. 29, 1838). The ear-labyrinth, it is
true, fuses indissolubly with the cranium at a later period, but the
bones which develop in its capsule are not for all that integral parts
of the primitive cranial vertebrae. This point, it is interesting to
note, had already been made by Oken in his _Programm_ (1807). But many
of the bones developed in relation to the sense organs can find their
place in the generalised embryonic schema or archetype of the vertebrate
skull, for they are of very constant occurrence during early
development.

Having arrived at a generalised embryonic type for the vertebrate skull,
of which the fundamental elements are the three cranial vertebrae and
their arches, Reichert goes on to discuss the particular forms under
which the skull appears in adult Vertebrates. He accepts in general von
Baer's law that the characters of the large groups appear earlier in
embryogeny than the characters of the lesser classificatory divisions.
"When we observe new and not originally present rudiments in very early
embryonic stages, as, for instance, that for the lacrymals, the
probability is that they belong to the distinctive development of one of
the _larger_ vertebrate groups. From these are to be carefully
distinguished such rudiments as arise later during ossification, mostly
as _ossa intercalaria_, in order to give greater strength to the skull
in view of the greater development of the brain, etc.; the latter give
their individual character to the _smaller_ vertebrate groups, and
comprise such bones as the _vomer_, the _Wormian bones_, the lowermost
turbinal, etc." (p. 63, 1838).

He did not accept the Meckel-Serres law of parallelism. He recognised
the great similarity between the unsegmented cartilaginous cranium of
Elasmobranchs, and the primordial cranium of the embryos of the higher
Vertebrates, but he did not think that the cranium of Elasmobranchs was
simply an undeveloped or embryonic stage of the skulls of the hig