THE SKULL. 



57 



c. 



Brain-Case (Cranium). 



The first cartilaginous rudiments appear in the primitively 

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

 trabeculse cranii. These He along the base of the brain, their 

 posterior part embracing the notochord, and they thus are divisible 

 into prochordal (anterior) and parachordal (posterior) regions 

 (Fi<r. 42, Tr). The parachordal tract may extend further along the 

 notochord as a direct backward growth of the trabeculse, 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 trabeculae project forwards 

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

 pituitary space (Fig. 42, PR). 



FIG. 42. FIBST CARTILAGINOUS RUDIMENTS OF THE SKULL. 



C, notochord ; PE, separate parachordal elements ; Tr, trabeeulae cranii ; PR, 

 pituitary space ; JV, 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 trabeculae 

 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 trabeculae 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.IS). 



