DEVELOPMENT OF THE BONES. 961 



tebral substances, in which the notochord persists, as the softer fibro- 

 cartilaginous centre. Whilst the bodies of the vertebrae are thus pro- 

 duced, another portion of the dorsal division of the vertebral plates 

 ascends towards the back of the embryo, grows around the vertebral 

 groove, and then forms the arches and spinous processes of the verte- 

 brae, so completing the vertebral column. These cartilaginous arches 

 first close in the dorsal part of the column, and then meet everywhere, 

 except opposite the coccyx, and sometimes at the lower end of the 

 sacrum; the non-formation or non-union of these arches constitutes 

 spina bifida. 



At the anterior or cephalic end of the embryo, as already mentioned, 

 the sides of the vertebral groove, composed of the medullary and ver- 

 tebral plates of the external and middle germinal layers, expand, and, 

 meeting above in the middle line, inclose a space which forms the 

 rudiment of the cranium and its contents. This is soon marked off 

 into three pairs of symmetrical sacs or dilatations, named the cerebral 

 vesicles, which, bending down towards the yolk, form the cephalic flex- 

 ure, or bend of the neck. In that part of the walls of these vesicles 

 which corresponds with the middle germinal layer, the cephalic capsule 

 or primordial skull is formed; this is at first membranous, then partly 

 cartilaginous, and ultimately bony. Into the floor of this cavity, the 

 anterior extremity of the cartilaginous notochord, or chorda dorsalis, 

 penetrates only a short distance, reaching, in the middle line, as far 

 forwards as the future sella turcica in the sphenoid bone, which lodges 

 the pituitary body. The base only of the primordial skull becomes 

 cartilaginous, like the bodies of the primitive vertebrae; the sides and 

 upper part of the cephalic capsule remain membranous. The primor- 

 dial skull does not divide into transverse segments, like the rudimen- 

 tary annular vertebrae. Its cartilaginous portion is developed into the 

 base and sides of the occipital bones or basi- and ex-occipitals, the alae 

 majores of the sphenoid or ali-sphenoids, and the pre-sphenoids and 

 orbito-sphenoids or alae minores, also into the turbinate bones, the 

 median portion of the ethmoid bone, and the cartilaginous nasal septum. 

 The vomer, however, is formed from a membrane, below the septum. 

 The upper part of the occipital bone or supra-occipitals, the parietal 

 bones, the squamous part of the temporal bones, the frontal bones, and 

 the nasal bones, are developed as distinct opercular bones, directly 

 from special centres of ossification in membrane, and not from cartilage. 

 The petrous and mastoid parts of the temporal bone, and the floor of 

 the tympanum, are developed from osseous centres in the basal carti- 

 lage ; the tympanic ring arises from a fibre-cartilage, specially con- 

 nected with the ear. The bones of the face, including the upper and 

 lower jaws, also the ossicles of the ear, the styloid process of the tem- 

 poral bone, and likewise the hyoid bony apparatus, are developed from 

 the so-called visceral or branchial arches, which are formed, as will be 

 hereafter described, from the lateral ventral plates of the cephalic por- 

 tion of the embryo. 



Opposite the trunk or body of the embryo, the outer or lateral part 

 of the ventral plates extends downwards from the vertebral plates, to 

 surround the hgemal cavity or future thoracico-abdominal chamber of 



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