STRUCTURE OF THE VERTEBRATES 



129 



chondrocranium. The student should again be reminded that the 

 typical chondrocranium is like that described in the first para- 

 graph, with the result that the roof of the skull is dermal in 

 origin. 



Ossification of the Chondrocranium. Bone replacement begins 

 in a number of centers, arranged in transverse series. Due to this 

 arrangement of the bones the skull was once considered as modi- 

 fied vertebrae, but embryology has dispelled this theory. Begin- 

 ning posteriorly there are four series of bones. 



Ptery go-quadrate 

 MeckeVs cartilage 

 Orbit 



Operculum 



Rostrum 



Fig. 65. The Chondrocranium and Dermal Skull of the Sturgeon (Order 



Chondrostei). (A) is the unossified chondrocranium. (B) shows the external 



appearance of the skull with the dermal bones overlying and completely 



covering the chondrocranium. 



1. The cartilage surrounding the foramen magnum ossifies as 

 four bones, the occipital complex. The ventral is the basioccipi- 

 tal, the lateral are the paired exoccipitals, and the dorsal is the 

 supraoccipital. 



2. In front of the occipital bones is the posterior sphenoid 

 complex composed of three bones: the ventral basisphenoid, and 

 the lateral paired alisphenoids. Recall the chondrocranium shape, 

 and it will be clear that the alisphenoids form the sides of the 

 skull. 



