COMPAPxATIVE ANATOMY. 



In most cases a median cartilaginous bar (intertrabecula) is formed 

 between the trabeculse in front, fusing with them, and forming the ethmo-nasal 

 septum. It occasionally projects forwards to form a rostrum. 



We must now follow further the processes of growth, taking as 

 a foundation the primary condition of things described above, in 

 which the trabeculse have united together in the middle line. The 



FIG. 43. DIAGRAMMATIC TRANSVERSE SECTIONS, OF THE HEAD IN EMBRYO 

 (A) STURGEONS, ELASMOBRANCHS, ANURA, AND MAMMALS ; (B) URODELES AND 

 SNAKES ; AND (C) CERTAIN TELEOSTEANS, LIZARDS, CROCODILES, CHELONIANS, 

 AND BIRDS. 



Tr, trabeculse cranii ; G, brain ; A, eyes ; Ps, parasphenoid ; IS, interorbital septum ; 

 F, frontal ; Olf, olfactory nerve. 



cartilaginous basal plate now comes into relations with the olfactory, 

 optic, and auditory organs by the formation of processes which 

 serve particularly in the case of the olfactory and auditory 



FIG. 44. SECOND STAGE IN THE DEVELOPMENT or THE PRIMORDIAL SKULL. 



C, notochord ; B, basilar plate ; T r , trabecula, which has united with its fellow in 

 front of the pituitary space to form the ethmonasal septum (S) ; Ct, cornu 

 trabeculse, and AF, antorbital process, which supports the olfactory organ (NK) 

 in front and behind ; 01, foramina for exit of the olfactory nerves from the 

 cranium ; PF, postorbital process of trabecula ; NK, nasal capsule ; A, eye ; 

 0, auditory capsule. 



apparatus to connect the skull with the cartilaginous sense-cap- 

 sules, and thus to act as supports for them. Thus an olfactory, 

 an orbital, and an auditory region are early differentiated. 



