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 | | | | | | |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.