THE SKULL 



67 



"head-cavity." Later, however, the visceral region became re- 

 latively shifted to a greater or less degree, especially in the anterior 

 part of the head, so that its segments no longer corresponded to- 

 those of the cranial region. Thus we find that the segmentation of 

 the nervous, muscular, and visceral parts of the head do not correspond 

 with one another. But although the segmentation of the visceral 

 portion of the skull has in the course of phylogeny reached a certain 

 degree of independence, and the cranial portion alone can be looked 

 upon as being made up of a series of somites, it must not be 

 forgotten that mesoblastic tissue extends from the head-somites 

 into the visceral arches, each of the two anterior of which still 

 contain a ccelomic cavity at a certain period of development. 



a. Brain-Case (Cranium). 



The first cartilaginous rudiments appear in the primitively 

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

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

 parts embracing the notochord ; they are thus divisible into pro- 

 chordal (anterior) and parachordal 

 (posterior) regions (Fig. 49), which 

 may be continuous with one another. 

 The parachordals soon unite to form a 

 lasilar plate, which grows round the 

 notochord dorsally and ventrally, and 

 thus early forms a solid support for 

 the hinder part of the brain. The 

 slender trabeculse project forwards and 

 enclose a space, which may be spoken 

 of as the primitive pituitary space 

 (Fig. 49). 



These structures may undergo 

 further development in many dif- 

 ferent ways in the various Vertebrate 

 groups : the trabeculse may become 

 completely united with one another 

 in the median line (Fig. 50, A), and 

 the connective-tissue of the oral 

 mucous membrane may become ossi- 

 fied to form a parasphenoid (B). In 

 other cases, the trabeculse may become 

 compressed and partly aborted owing 

 to the great development of the eyes : 

 this obtains, e.g., in certain Reptiles 

 and in all Birds, in which a fibro-cartilaginous interorbital septum 

 appears in their place (C). 



In most cases a median cartilaginous bar (intertrdbeetda) is 

 formed between the trabeculse in front, fusing with them, and 



F 2 



FIG. 49. FIRST CARTILAGINOUS 

 RUDIMENTS OP THE SKULL. 



C, notochord ; PE, separate 

 parachordal elements ; Tr, 

 trabeculse cranii ; PR, pitui- 

 tary space; ^V, A, 0, the 

 three sense-capsules (olfac- 

 tory, optic, and auditory). 



