el. K. E. von Baer[224] in 1826 discussed at some length the relation
between the bony and the cartilaginous skull in fishes, with particular
reference to the sturgeon, coming to the following just conclusion:--"If
we consider the fibrous skeleton of _Ammocoetes_ as the first foundation
of the skeleton of Vertebrates, we can form a series among the
cartilaginous fishes, according as a cartilaginous skeleton penetrates
more and more into this fibrous foundation. In the same way the process
of ossification supplants the cartilaginous skeleton. So long as the
ossifications lie in the skin, as in the sturgeon, they form corneous
bones (_Hornknochen_), but when they lie under the skin, they form true
bones, _e.g._, the bones of the skull in the pike" (p. 374).

Embryologists soon become aware that a similar distinction between a
primitive cartilaginous foundation and a secondary overlying
ossification of the skull showed itself in the development of all
Vertebrates. Duges, in his _Recherches sur l'osteologie et la myologie
des Batraciens_ (1834), distinguished between such bones as are formed
by direct ossification of the cartilaginous groundwork of the skull, and
such as are developed in the periosteal fibrous tissue.

Reichert in 1838[225] noted that several of the skull bones in Amphibia
are formed without the intermediary of cartilage, such as the nasals,
the maxillaries and the lacrymals. So, too, the frontals and parietals
of Teleosts developed independently of the cartilaginous skull, and
belonged to the skeletal system of the skin, not to the true vertebral
axial skeleton (pp. 215-6). Even more interesting was his discovery,
afterwards confirmed by Hertwig,[226] that in the newt several bones
connected with the palate were formed in the mucous membrane of the
mouth by the fusion of a number of little conical teeth (p. 97). Certain
of these bones he considered to be the substitutes, not the equivalents,
of the palatine and pterygoid of other Vertebrates, which are formed
from the upper part of the first visceral arch, a part missing in the
newt (p. 100). Owing to the difference of development he would not
homologise these bones in the newt with the palatine and pterygoid of
other Vertebrates. He recognised also that the bone now known as the
parasphenoid was developed in the frog in the mucous membrane of the
mouth, and had originally no connection with the cranial basis (p. 34).
Rathke in 1839 also allowed the distin