199 



3. The wide auditory region. 



4. The posterior or occipital region surrounding the 



foramen magnum, through which the spinal 

 cord is continuous with the brain. This region 

 is either united to or articulated with the 

 vertebral column. Besides the foramen magnum, 

 there are smaller openings in the walls for the 

 passage of the cranial nerves. 



Such is the primary formation of the skull ; in this 

 cartilaginous state it remains throughout life in the lowest 

 Craniate? (e.g., Cyclostomata and Elasmobranch fishes), 

 but in higher Craniates that condition is temporary, the 

 cartilage being strengthened or replaced (to a varying 

 extent, never entirely) by a secondary development of bone. 

 In the salmon the chondrocranium is largely supplemented 

 and replaced by bone ; and in the cod only remnants of the 

 original cartilages remain. 



The open parts (fontanelles) of the skull roof are in 

 Elasmobranchs covered only by the tough integument, 

 which often bears placoid scales or skin teeth of dermal 

 bone. The sturgeon has on the top of the head, above 

 the chondrocranium, large bony plates. 



Several of the skull bones are of dermal origin ; some of 

 these retain their dental character, and cover the cartilage 

 superficially (e.g., vomer and palatine of roof of mouth, 

 and premaxilla, maxilla, and dentary of the jaws) ; others 

 become investment bones, and are in close contact with the 

 cartilage (e.g., nasal, frontal and parietal of the skull roof). 

 The substitution or replacement bones (e.g., the occipital 

 and sphenoid) replace the cartilage which is destroyed ; 

 and these are often termed cartilage bones, the other two 

 kinds being the so-called membrane bones. The skull of 

 the higher Craniate is therefore of mixed origin. 



Write an Account of the Visceral Arches. 



The visceral arches, a series of (from 4 to 9) hoop-like 

 cartilages, are closely connected with the skull, and form 

 the supporting framework of the slit walls of the branchial 

 portion (pharynx) of the alimentary canal. In that region, 

 on each side, cartilaginous rods are developed (from mesen- 

 chyme) between the gill-clefts and around the mouth. 

 In gill- breathing fishes, each of these rods is typically, 

 composed of four segments ; the rods are united in pairs 

 by ventral pieces, and the successive arches, thus formed, 



