ction between cartilage and
membrane bone, but laid no stress upon it (_Entw. d. Natter._, p. 197).

Jacobson in 1842[227] introduced the useful term, "primordial cranium,"
for the primitive cartilaginous foundation of the skull, and drew a
sharp distinction between cartilage bones and membrane bones.

In his _Recherches sur les Poissons fossiles_,[228] L. Agassiz used Vogt's
work on the development of _Coregonus_ to establish a classification of
the bones of the skull in fish, a classification which had the merit of
drawing a sharp distinction between the cartilaginous groundwork and
the "protective plates" of the fish's skull. He recognised that the
protective plates developed in a different way from the other bones of
the skull. "We must distinguish," he writes, "two kinds of ossification;
one which tends to transform the primitive parts of the embryonic
cranium directly into bone, and another which leads to the deposition of
protective plates round this core, which develop not only upon the upper
surface, as has hitherto been supposed, but also on the lateral walls
and on the lower surface of the cranium" (p. 112). In the skull of all
fish there are three elements--(1) the cartilaginous base, including the
nuchal plate, the trabeculae and the facial plate, together with the
auditory capsules; (2) the cartilaginous cerebral envelope; (3) the bony
protective plates (absent in Elasmobranchs). The bones developed in
relation to these cranial elements can be classified as follows:--(1)
the basioccipital, exoccipitals (paroccipitals?), supraoccipital and
"petrous" (_rocher_), developed from the nuchal plate; the ali- and
orbito-sphenoids developed from the trabeculae; the "cranial ethmoid"[229]
developed from the facial plate; (2) the parietals, frontals and nasals
formed from the "superior" protective plate; the "anterior" and
"posterior" frontals and the temporal, from the "lateral" plates; the
body of the sphenoid and the vomer from the "inferior" plates. The other
element, the cartilaginous brain-box, does not ossify, and tends to
become absorbed (p. 124).

In 1849 Koelliker published a paper[230] dealing with the morphological
significance of the distinction between membrane and cartilage bones,
and in 1850[231] he defended his views against the criticisms of
Reichert[232] in a further note entitled _Die Theorie des
Primordialschaedels festgehalten_. It is convenient to consider these
papers together. Koelliker hel