SKULL 



79 



region. Thus a continuous cartilaginous neurocranium is formed 

 such as persists throughout life in Elasmobranchs, for example. 

 But in by far the greater number of Vertebrates, the cartilage 

 does not play so great a part, and is, as a rule, confined to the base 

 and lower parts of the sides of the skull and to the sense capsules, 

 except in the occipital region, where it always extends over the 

 brain. The rest of the skull, more particularly the roof, becomes 



Hum'/ 



...ttnl. raps. 



/s; 



fj Columella 



J#% 



Notochora 



FIG. 61. DIAGRAM OF A TROPIBASIC PRIMORDIAL SKULL, BASED ON THE 

 CONDITIONS SEEN IN THE AMNIOTA, AND SHOWING THE RELATIONS OF THE 

 MORE IMPORTANT FORAMINA. (After E. Gaupp. ) 



The cerebral nerves or their foramina are indicated by Roman numerals : Sp. o<\, 



foramina for spino-occipital nerves. 



directly converted from membrane into bone (investing bones) : at 

 the same time, bones may become differentiated in connection with 

 the chondrocranium itself, which is thus more or less completely 

 replaced by an osteocranium. In general, the higher the systematic 

 position of the animal, the less extensive are the cartilaginous 

 constituents and the more important the bony elements. 



The investing bones (allostoses) have originally no direct con- 

 nection with the chondrocranium, and thus may be contrasted with 

 those bony elements which are formed in close relation with it, 



