A. The “vertebral doctrine” starts from the undeniable fact that the skull is originally composed of several segments, each of which is merely the modification of a vertebra. The component parts of such a cranial segment are considered to be homologous to those of a vertebra. Three, four, or five cranial vertebræ have been distinguished, all the various bones of the fully-developed and ossified skull being referred, without distinction as to their origin, to one or the other of those vertebral segments. The idea of the typical unity of the osseous framework of Vertebrates has been worked out with the greatest originality and knowledge of detail, by _Owen_, who demonstrates that the fish-skull is composed of _four_ vertebræ. The bones of the fish-skull are, according to him, primarily divisible into those of the _neuroskeleton_, _splanchnoskeleton_, and _dermoskeleton_. The bones of the _neuro_- or proper _endoskeleton_ are arranged in a series of four horizontally succeeding segments: the occipital, parietal, frontal, and nasal vertebræ; each segment consisting of an upper (neural) and a lower (hæmal) arch, with a common centre, and with diverging appendages. The _neural_ arches of the four vertebræ, in their succession from the occiput towards the snout, are:-- 1. _Epencephalic arch_, composed of the occipitals. 2. _Mesencephalic arch_, composed of basisphenoid, alisphenoid, parietal, and mastoid. 3. _Prosencephalic arch_, composed of presphenoid, orbito-sphenoid, frontal, and postfrontal. 4. _Rhinencephalic arch_, composed of vomer, prefrontal, and nasal. The _hæmal_ arches in the same order of succession are:-- 1. _Scapular_ or _scapulo-coracoid_ arch, composed of suprascapula, scapula, and coracoid; its appendage consists of the ulna, radius and carpal. 2. _Hyoid_ or _stylo-hyoid_ arch, composed of stylohyal, epihyal, ceratohyal, basihyal, glossohyal, and urohyal; its appendage is the branchiostegals. 3. _Mandibular_ or _tympano-mandibular_ arch, composed of epi-, meso-, pre-, and hypo-tympanic, and the bones of the lower jaw; its appendage consists of the præoperculum and the other opercles. 4. _Maxillary_ or _palato-maxillary_ arch, composed of palatine, maxillary, and premaxillary; its appendage consists of the pterygoid and entopterygoid. Parts of the _splanchnoskeleton_ are held to be the ear-capsule or petrosal and the otolite, the eye-capsule or sclerotic, the nose-capsule or “ethmoid” and turbinal; the branchial arches. The bones of the _dermoskeleton_ are the supratemporals, supraorbitals, suborbitals, and labials.