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 Ammnzota 
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) Zhe Visceral Arches.—The first two cartilaginous 
visceral arches of the fishes are called the mandibular and 
the Ayoid. Each has an upper and lower half on each side. 
The upper half of the mandibular arch is called the padazo- 
guadrate 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 Ayomandibular cartilage which 
is attached to the otic or ear-region of the skull; the lower 
is the Ayoid 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 palatoguadrate cartilage is replaced 
by the palatines, pierygoids and guadrates, and, in addition, 
the premaxille, maxille 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 Zé/eostomi, and in all fishes they 
serve as a support to the gills and walls of the pharynx. In 
the Ammniofa they mainly disappear. The first branchial 
usually remains in part as the posterior cornu of the hyoid, 
M. 28 
