THE SKULL. 



a. Brain-Case (Cranium). 



The first cartilaginous rudiments appear in the primitively 

 membranous skull tube in the form of a pair of rods, the 

 trabeculge cranii. These lie along the base of the brain, their 

 posterior part embracing th'e notochord, and the} 7 thus are divisible 

 into prochordal (anterior) and parachordal (posterior) regions 

 (Fig. 42, Tr). The parachordal tract may extend further along the 

 notochord as a direct backward growth of the trabecula?, or as one 

 or two separate cartilaginous tracts (Fig. 42, PE). The para- 

 chordals soon unite to form a basilar plate, which grows round 

 the notochord dorsally and ventrally, and thus early forms a solid 

 support for the brain. The slender trabeculas project forwards 

 and enclose a space, which may be spoken of as the primitive 

 pituitary space (Fig. 42, PE). (':Q 



Fia. 42. FIRST CARTILAGINOUS RUDIMENTS or THE SKULL. 



C, notocho^d ; PE, separate paracliordal elements ; Tr, trabeculse cranii ; PR, 

 pituitary space ; N, A, 0, the three sense-capsules (olfactory, optic, and 

 auditory). 



These structures may become further developed in many 

 different ways in the various Vertebrate groups : either the trabeculse 

 become completely united with one another in the median line 

 (Fig. 43, A, Tr), or the connective- tissue of the oral mucous mem- 

 brane becomes ossified to form a parasphenoid (Fig. 43, B, Ps). 

 In other cases, the trabeculse may become compressed and partly 

 aborted owing to the great development of the eyes ; this obtains 

 in certain Teleosteans and Reptiles and in all Birds, where a fibro- 

 cartilaginous interorbital septum appears in their place (Fig. 43, 

 C, Tr, />Sf). 



