68 



COMPARATIVE ANATOMY 



forming the ethmo-nasal septum (Fig. 51). It occasionally projects 

 forwards to form a rostrum (Figs. 55, 56, and 58). 



FIG. 50. DIAGRAMMATIC TRANSVERSE SECTIONS OF THE HEAD IN EMBRYO 

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

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

 AND BIRDS. 



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

 septum ; F, frontal ; Off, olfactory nerve. 



We must now further follow the processes of growth, start- 

 ing from the primary condition described above, in which 



the trabeculse have united together in 

 the middle line. The cartilaginous 

 basal plate now comes into relations 

 with the olfactory, optic, and auditory 

 organs (Fig. 51), around which carti- 

 laginous capsules are formed. Thus an 

 olfactory, an orbital, and an auditory 

 region are early differentiated. 



The olfactory and auditory capsules, 

 especially in higher types, then become 

 more and more drawn in to the skull 

 proper, and the lateral edges of the 

 basal plate begin to grow upwards 

 round the brain on both sides, eventu- 

 ally extending even to the dorsal region. 

 Thus a continuous cartilaginous capsule 

 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, con- 

 fined 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 directly converted 

 from membrane into bone. At the 

 same time, bones may become differ- 

 entiated in connection with the primary 



FIG. 51. LATER STAGE IN THE 

 DEVELOPMENT OF THE PRIM- 

 ORDIAL SKULL. 



C, notochord ; B, basilar plate ; 

 Tr, trabecula, which has 

 united with its fellow in 

 front of the pituitary space 

 to form the ethmo-nasal sep- 

 tum (S) ; Ct, cornu trabe- 

 culse, and AF, antorbital 

 process, which support the 

 olfactory organ (NK) in 

 front and behind ; Ol, for- 

 amina for exit of the olfac- 

 tory nerves from the crani- 

 um ; PF, postorbital pro- 

 cess of trabecula ; A , eye ; 

 O, auditory organ. 



