84 



OSTEOLOGY 





Fossa 

 hypophyseos 



Mesoderm of base 

 of skull 



Parachordal 

 cartilage 



The notochord extends as far forward as the anterior end of the mid-brain, and 

 becomes partly surrounded by mesoderm (Fig. 68) . The posterior part of this meso- 

 dermal investment corresponds with the basilar part of the occipital bone, and shows 

 a subdivision into four segments, which are separated by the roots of the hypo- 

 glossal nerve. The mesoderm then extends over the brain-vesicles, and thus the 



entire brain is enclosed by a mesodermal 

 investment, which is termed the membran- 

 ous cranium. From the inner layer of this 

 the bones of the skull and the membranes 

 of the brain are developed ; from the outer 

 layer the muscles, bloodvessels, true skin, 

 and subcutaneous tissues of the scalp. In 

 the shark and dog-fish this membranous 

 cranium undergoes complete chondrifi- 

 cation, and forms the cartilaginous skull 

 or chondrocranium of these animals. In 

 mammals, on the other hand, the process 

 of chondrification is limited to the base 

 of the skull the roof and sides being 

 covered in by membrane. Thus the bones 

 of the base of the skull are preceded by 

 cartilage, those of the roof and sides 

 by membrane. The posterior part of the 

 base of the skull is developed around 

 the notochord, and exhibits a segmented 



condition analogous to that of the vertebral column, while the anterior part arises 

 in front of the notochord and shows no regular segmentation. The base of the skull 

 may therefore be divided into (a) a chordal or vertebral, and (6) a prechordal or 

 prevertebral portion. 



Anterior arch of atlas 



Notochord 

 Body of axis 



Third cervical 

 vertebra 



FIG. 68. Sagittal section of cephalic end of noto- 

 chord. (Keibel.) 



Situation of olfactory pit Ethmoid plate 



and nasal Olfactory organ 

 septum. 



Fossa 

 hypophyseos 



Trabecula 

 cranii 



Situation of 

 auditory _ 



Parachordal" 



cartilage 

 Notochord' 



Extension around 

 * olfactory organ 

 Foramina for 

 olfactory nerves 



Eyeball 



Fossa 

 hypophyseos 



Basilar plate 

 -"Auditory vesicle 



Notochord 



FIG. 69. Diagrams of the cartilaginous cranium. (Wiedersheim. 



In the lower vertebrates two pairs of cartilages are developed, viz., a pair of 

 parachordal cartilages, one on either side of the notochord; and a pair of pre- 

 chordal cartilages, the trabeculse cranii, in front of the notochord (Fig. 66). The 

 parachordal cartilages (Fig. 69) unite to form a basilar plate, from which the car- 

 tilaginous part of the occipital bone and the basi-sphenoid are developed. On the 

 lateral aspects of the parachordal cartilages the auditory vesicles are situated, 



