which later the skeleton of the visceral arches
developed. The walls of the upper tube formed the bones and muscles of
the cranium proper. The facial part of the head was formed by elements
from both upper and lower tubes. The dorsal tube showed signs of a
division into three cranial vertebrae (_Urwirbeln_, primitive vertebrae).
In mammals and birds, as Reichert had shown in his 1837 paper, the three
cranial vertebrae were indicated by transverse furrows on the ventral
surface of the still membranous skull (see Fig. 10, p. 148).

Even in mammals and birds, however, the positions of the eye, the
ear-labyrinth, and the three visceral arches were the safest guides to
the delimitation of the cranial vertebrae (pp. 134-138, 1837). In
Amphibia generally there were no definite lines of separation on the
skull itself. "At this stage," he writes of the cartilaginous cranium of
the frog, "we find no trace of a veritable division into vertebrae in the
cartilaginous trough formed by the _basis cranii_ and the side parts. On
the contrary, it is quite continuous, as it is also in the higher
Vertebrates during the process of chondrification" (p. 44, 1838). The
vertebrae in the membranous or cartilaginous skull could be delimited in
Amphibia by the help of the eye and the ear-labyrinth, which lie more or
less between the first and second, and the second and third vertebrae,
but, above all, by the vesicles of the brain.

As in the higher Vertebrates, the visceral arches are associated with
the cranial vertebrae as their ventral extensions, being equivalent to
the visceral plates which form the ventral portion of the "primitive
vertebrae" or primitive segments of the trunk.

[Illustration: FIG. 10.--Cranial Vertebrae and Visceral Arches in Embryo
of Pig. Ventral Aspect. (After Reichert.)]

If the three cranial vertebrae are not very distinct in the early stages
of development when the skull is still membranous or cartilaginous, they
become clearly delimited when ossification sets in. Three rings of bone
forming three more or less complete vertebrae are the final result of
ossification. The composition of these rings is as follows:--

+-------------------------------------------------------------------+
| | Base. | Sides. | Top. |
|----------------+---------------+-----------------+----------------|
|First vertebra |Presphenoid |Orbitosphenoids |Frontals |
| |