SKULL 77 



(Figs. 58 and 59), which may be continuous with one another. 

 They lie along the base of the brain, the parachordals embracing 

 the anterior end of the notochord. The parachordals soon unite, 

 more or less completely, to form a basal plate, which grows round 

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

 support for the hinder part of the brain. The trabecula3 project 

 forwards and enclose a space, which, as the pituitary body extends 

 from the brain through its posterior part, may be spoken of as the 

 primitive pituitary space (anterior basicranial fontanelle). In the 

 parachordal region, an anterior auditory or otic, and a posterior 

 occipital portion, may be recognised on either side. The occipital 

 region, as already mentioned, may show indications of segmentation 



Occipital arch 



Comu trabecuta IT/ R|Kf J^ ^"^ Ifotochord 



Auditory capsule 

 Otic process 

 Palatoquadratt 

 Articular process 



FIG. 59. NEUROCRANIUM AND PALATOQUADRATE OF LARVAL AMBLYSTOMA, 

 9 MM. IN LENGTH, SEEN OBLIQUELY FROM THE LEFT SIDE AND ABOVE. 

 x ABOUT 35. (From copy by Fr. Ziegler of a model by Ph. Stohr. ) 



II, foramen for optic nerve, and III, for oculomotor nerve. 



in the nerve-apertures which perforate the cartilage as well as in 

 the surrounding myomeres : its anterior limit is marked by the 

 vagus nerve. 



Around the auditory organ of either side is developed, usually 

 independently, a cartilaginous auditory capsule (Figs. 59-61), 

 which is relatively larger in the lower than in the higher Vertebrates. 

 It is situated laterally to the otic region, between the trigeminal 

 and vagus nerves, and becomes closely connected with the corres- 

 ponding parachordal which may help to complete it. 



The trabeculae may remain separated from one another or may 

 become united along the greater part of their length under the 

 influence of the developing optic capsules. In the former case the 

 skull may be described as platybasic, and the brain extends for- 

 ward interorbitally to the ethmoidal region (many Elasmobranchii, 

 Dipnoi, Amphibia, Fig. 60) ; in the latter, or tropibasic type (Fig. 

 61), the trabeculaB give rise to a thin interorbital septum, and the 



