414 CHORDATA. 
say in the mechanical centre, and thence to form bone to 
either end. But bone is a harder and'more resistant substance 
than cartilage. and it is often more to the advantage of the 
organism that the parts which are subjected to special strain 
should first be ossified. Hence the ends of the long bones, 
which form the joints, and very often other parts, such as 
trochanters and tuberosities acting as points of attachment 
for muscles or tendons, have separate centres of ossification. 
When the cartilage ceases to grow, then the ossification 
proceeding from each centre, the bony elements meet and a 
single complete bone results. Jf, as in mammals, the bony 
elements are separated for a long time bya thin layer of 
growing cartilage, then the elements are separated in the 
dried skeleton by “sutures” and may fall apart. Hence 
the caps or epiphyses already referred to. But the final 
result is a single bone of the same size and shape as the 
cartilage. 
In many cases the single piece of cartilage may be re- 
placed permanently by two or more bones with a joint 
between them. Cartilage is elastic, and a piece of cartilage 
may therefore “ give” to certain strains, by virtue of its elas- 
ticity, sufficiently to dispense with the necessity for a joint. 
Bone, however, is far more rigid, and hence a single elastic 
cartilage, such as the palatoquadrate bar, is replaced by at 
least three bones—the palatine, pterygoid and quadrate— 
which are more or less movable on each other. The replace- 
ment of the hyomandibular cartilage of lower types by three 
(or four) ossicles of the ear is probably another instance. 
The skeleton can be conveniently considered under two 
heads :—1. The axial skeleton, skull and vertebre. 2. The 
appendicular skeleton, limbs and limb-girdles. 
AxtaL.—The skull has a double origin, being really 
formed of two parts which are almost entirely distinct in 
the fishes. These are (1) cranium; (2) the visceral arches. 
The cranium arises essentially as a protecting mass to 
the underlying brain, and the visceral arches arise primarily 
as strengthening bars between the branchial clefts. The 
first two of these arches alone take any part in the formation 
of the skull. 
(1) The Cranium.—tIn the earliest stages the brain is 
enclosed on all sides by a membranous sheath which also 
