I 



I 



VERTEBRATA. 417 



were to commence at one part of the skull, say the hind-end, and work 

 forwards, the one part of the skull would become ossified too soon to 

 allow the necessary growth in size, and the rest would ossify too late to 

 form an efficient protective cranium. Hence ossification begins at 

 various points simultaneously. These points are called centres of ossifi- 

 cation., and their position is determined by mechanical conditions. 

 Radiating in all directions from these centres, each bone is gradually 

 produced until it comes to touch its fellow. Hence the ossification of 

 the skull is effected by "piecework," divided amongst the centres of 

 ossification ; and in the cranium, in which a general protective function 

 is requisite, the "pieces" are divided fairly accurately into successive 

 rings, each of which is again subdivided into three or four. Thus we 

 seek to explain the "segmental" formation of the bony skull as due 

 rather to an orderly mechanical method of producing an osseous cranium 

 from cartilage, than as indicating a primary origin of the skull from 

 vertebrae. The method of ossification of a vertebra is due to a similar 

 cause. 



If all the cartilage becomes ossified, a continuous bony cranium is 

 the result, incapable of further increase in size ; but in most Avuiiota 

 the bones remain for a long time (until late in life) separated by a thin 

 layer of growing cartilage which leaves a "suture" in the dry skull. 

 This enables every bone to continue increasing in size and with them 

 the entire cranium. 



(2) The Visceral Arches. — The first two cartilaginous 

 visceral arches of the fishes are called the ma?idibular and 

 the hyoid. Each has an upper and lower half on each side. 

 The upper half of the mandibular arch is called the palato- 

 quadrate bar and the lower the mandible^ and the two are 

 bent upon each other to form upper and lower jaw. The upper 

 half of the hyoid arch is the hyomandibtilar cartilage which 

 is attached to the otic or ear-region of the skull ; the lower 

 is the hyoid cartilage. These visceral arches are attached by 

 ligament to the cranium in the lower types, but in the higher 

 the bones which replace them form the very important facial 

 part of the skull. The palatoquadrate cartilage is replaced 

 by the palatines^ pterygoids and quadrates^ and, in addition, 

 the premaxillce, maxillce and Jugals are added in connection 

 with it. The mandibular cartilage is replaced by the 

 mandible and the hyomandibular cartilage by the hyo- 

 mandibular bone. 



The succeeding arches are called branchial arches. There 



are five in the skate, four in Teleostomi, and in all fishes they 



serve as a support to the gills and walls of the pharynx. In " 



the Amniota they mainly disappear. The first branchial 



jually remains in part as the posterior cornu of the hyoid, 



M. 28 



