34 THE FROG 
ment of cartilage are called cartilage bones. By far the greater 
portion of the skeleton of the frog is made up of cartilage bones. 
Cartilage may also be calcified (1. e., become hardened by a deposit 
of calcareous material) without taking on the histological char- 
acter of true bone. 
Bones may also be developed in places where there is no pre- 
existing cartilage, and are then, in distinction from cartilage 
bones, called membrane bones. Membrane bones are formed as 
ossifications in the cutis or deeper layer of the skin; in many fishes 
they retain this primitive position, but in the frog and in most 
higher vertebrates they sink beneath the skin and come into closer 
relation to the more deeply placed cartilaginous skeleton. In the 
frog many of the bones of the skull, and in addition the clavicles, 
are membrane bones. 
With a razor or a sharp scalpel cut off a thin slice of the un- 
calcified (clear or hyaline) cartilage from the proximal end of 
the femur, mount it on a slide in a drop of water and cover it 
with a cover glass; examine it first with the low and then with 
the high power of the microscope. Observe the clear matrix and 
the cell spaces each containing one or more cells. Sketch a por- 
tion, representing each cell at least one centimeter in diameter. 
Study prepared slides showing cross sections of the femur of . 
the frog, verifying so far as possible the description on pp. 126— 
128 of Holmes’ Biology of the Frog. Sketch a sufficient portion 
of the section to show the various structures seen; label all parts. 
Examine a cross-section of dry bone taken from some mammal; 
how does it differ in structure from the bone of the frog? 
The various parts of the skeleton are either immovably fastened 
together, or connected in such a way as to allow some freedom of 
motion. In either case the connection is called an articulation or 
joint. Where slight movement is required, the bones are united by 
pads of tough and elastic fibro-cartilage, as in the joints between 
the bodies of the vertebrae; in the more movable joints, each 
articular surface is covered with a pad of cartilage, and the bones 
are held together by strong bands or capsules of fibrous material, 
called ligaments. 
With the aid of the following outline, study both entire and 
disarticulated skeletons of the frog, and be prepared to demon- 
