SKULL — OSSIFICATION 



69 



Cartilage Bones. — All cartilage bones are laid down as small 

 centres, either on or in the cartilage (peri- or endochondrostoses, 

 p. 7) with no periosteum or perichondrium between bone and 

 cartilage. Sometimes there are two centres in what is here treated 

 as a single bone (basi- and presphenoids, etc.) . First to be considered 

 are the bones of the cranium, those of the visceral arches following. 



The base (hinder end) of the chondrocranium is perforated by 

 a large opening, the foramen magnum, through which the brain 

 connects with the spinal cord. Around this opening four cartilage 

 bones, grouped as occipitalia, are developed. These include a 



Fig. 73. — Diagram of ventral side of skull; chondrocranium shaded. Cartilage 

 stippled; cartilage bones with lines and dots, membrane bones outlined. 



basioccipital below the foramen, ossifying in the hinder part of the 

 basal plate, the notochord passing through it in the early stage. On 

 either side of the foramen is an exoccipital (pleuroccipital), formed in 

 what would be the neurapophyses of this vertebral part of the 

 cranium. The ring is completed above by a supraoccipital, formed 

 in the synotic tectum and the apex of the vertebral arch. The 

 supraoccipital is the only cartilage bone in the cranial roof of any 

 vertebrate. 



The sphenoidalia (fig. 73) arise in front of the occipitaha. Of 

 these there are two groups, the basisphenoid group just in front of 

 the occipitals, the presphenoid group in front of the other. The 



