152 



TIIK EMBRYOLOGICAL CRITERION 



(p. 40). When ossification sets in, the ear-capsule forms 

 three bones, of which two fuse with the supraoccipital and 

 exoccipitals. 



During the formation of the ear-capsule the cranial basis 

 develops from a plate to a trench, for in its hinder section 

 the side parts grow up to form the side walls of the brain, in 

 exactly the same way as the processes of the vertebral 

 rudiments grow up to enclose the spinal column (pp. 122, 

 192). The foundations of the skull are now complete, and 

 ossification gradually sets in. The basioccipital is formed 



FIG. ii. Embryonic Cranium of the Adder. Ventral 



(After Rathke.) 



a. Basioccipiul. 

 '-. Kxoreipital. 

 c. Ear capsule. 



('. Bssisphenoid. 



c. Alispheiioid. 

 /. OrbitosphenoiJ. 



Ii. Foramen. 

 . I'ituilar.v space. 



in the posterior part of the basis cran'ii, and the exoccipitals 

 in the side walls of the trench in continuity with the 

 fundament of the basioccipital (see Fig. 11). The supra- 

 occipital is formed in cartilage above the exoccipitals. The 

 basisphenoid develops, like the basioccipital, in the flat basis 

 era nii, but towards its anterior edge, between the large 

 foramen (//) and the pituitary space (/). It is formed from 

 two centres, each of which is originally a ring round the 

 carotid foramen. The presphenoid develops in isolation 

 between the lateral trabeculae, just behind the point where 



