XIII 



SKULL 



605 



ctr 

 o/ 



extends into the head as far as the pituitary body (Fig. 

 163). The cranial cartilages do not become segmented, 

 but give rise to a pair of horizontal bars, the parachordals 

 (pa. ch) : these are continued forwards, diverging around 

 the pituitary body, as the trabeculcz cranii (tr). The 

 two parachordals and 

 trabeculae then unite 

 respectively with one 

 another, and so form 

 a firm floor for the fu- 

 ture brain-case : this is 

 gradually developed by 

 the cartilage 'growing 

 up on either side and 

 eventually meeting to 

 a greater or less extent 

 above the brain : there 

 is never, however, a 

 complete cartilaginous 

 roof to the cranium, 

 parts of which are 



Only membranOUS and FIG. ^63. Dissection of the head of an embryo 



form the fontanelles 

 (pp. 43 and 436). 

 In the meantime the 

 cartilaginous sense- 

 capsules are deve- 

 loped, the olfactory 

 and auditory capsules 



uniting with the brain-case in front and behind 

 respectively. 



The visceral skeleton is formed as a series of cartila- 

 ginous bars within the visceral arches, the first of which 

 forms the mandibular arch, the second the hyoid, and 

 the others the branchial arches. 



Dogfish (Scyllium canicula) from the dorsal 

 side, to show the developing chondro- 

 cranium (dotted), (x 8.) 

 au. auditory capsule ; br. external gills ; 

 c.tr. cornu of trabecula ; cfi, c/ 4 . gill-clefts ; 

 inf. infundibulum of brain (left in situ) ; 

 nc. notochord ; ol. olfactory sac ; pa. ch. 

 parachordal cartilage ; pn. prenasal cartilage ; 

 py. pituitary body (left in situ) ; sp. spiracu- 

 lar cleft ; ' 

 Parker.) 



tr. trabecula. (After W. K. 



