THE SKELETON OF THE FROG. 45 
Chap. IV.—THE SKELETON OF THE FROG. 
The skeleton, which forms the hard internal parts of the frog, 
is composed partly of cartilage and partly of bone. It forms a 
framework giving definite shape to the body, and precision to 
the movements ; and serves also to protect from injury some of 
the more important and delicate organs, notably the central 
nervous system, the sense organs, and the heart. In the early 
stages of its development the skeleton consists entirely of 
cartilage: in the adult this primary cartilaginous skeleton is 
replaced to a greater or less extent by bone. Bone may also be 
developed in places where there was no pre-existing cartilage, 
and is then called membrane bone, in contradistinction to the 
former kind, or cartilage bone, which replaces cartilage. 
Membrane bones arise in the first instance as ossifications in the 
dermis or deeper layer of the skin: in many fish they retain 
this primitive position, but in the frog and most higher verte- 
brates they sink below the skin and graft themselves on to the 
more deeply placed cartilaginous skeleton. Cartilage may also 
become calcified, z.e., have calcareous salts deposited in its 
matrix, without in any way taking on the character of true bone. 
The skeleton may be conveniently divided into (1) the axial 
portion, including the skull and the vertebral column: and 
(2) the appendicular portion, including the limbs and the 
limb-girdles which attach them to the body. 
Examine the prepared skeletons and make careful drawings to 
scale of the several parts. Colour, in your drawings, the cartilage 
blue, the cartilage bones yellow, and the membrane bones white or 
red. Prepare skeletons for yourself by soaking the parts in hot 
water, and carefully brushing away the soft tissues until the 
skeleton ts clean. 
