SKULL 



255 



an anterior related to the auditory capsule and a posterior or occipital segment. The otic portion 

 shows no trace even in the membranous stage of any segmentation ; but regarding the occipital 

 portion, Levi confirms for man the accounts given by Froriep for the calf and recently by Weiss 

 for the rat. Froriep holds that the occipital region represents the fusion of four rudimentary 

 vertebrae, corresponding to the three primary roots of the hypoglossal nerve. Of these only the 

 posterior is at all independent. Its development in the early stages resembles that of the vertebrae, 

 and it loses its identity only when fused with the parts in front of it. The anterior portion of 

 the occipital blastema shows faint traces of segmentation, but only in the earliestjphases, by 



rist a gulli 



lamina cribrosa 



ala orbit alis 



for. oplicum 



'tla temporalis 



for. ami. int. 

 for.jiKjul. 



fossa subarciiata 



can. nervi 



fadalis 



udit. caps. 



for. endol. 



for. liupogloss 



foramrn magnum lectum siinoticmn 



FIG. 309. MODEL OF THE CHONDROCRANIUM OF A HUMAN EMBRYO, 8 CM. (From Hertwig's 

 Handbuch der Entwickelungslehre.) 



The membrane-bones are not represented. 



the presence of three rudimentary cellular masses (primitive arches) which extend outwards 

 between the myotomes. After the cartilaginous elements are formed, all trace of segmentation 

 is lost. The relation of the basicranial plate to the notochord (see below) in mammals makes 

 the significance of this early segmentation doubtful (Robinson). 



The chondrocranium forms only an incomplete case for the brain. The cranial 

 vault consists of bones which are laid down directly in membrane, and in the 

 region of the visceral skeleton numerous investing bones are added to complete 



