126 THE BIOLOGY OF THE FROG chap. 



tilages are covered by a layer, or perichondrium, which 

 consists of an outer fibrous membrane, below which are con- 

 nective tissue corpuscles, which, as the cartilage grows, sink 

 into the matrix and become transformed into ordinary car- 

 tilage cells. Hyaline cartilage occurs at the ends of the 

 ^i, bones of the limbs, be- 



^ '^' /■•r^ tween the vertebrse and 



ry ,^ ?/' ' ^V9 at the ends of their trans- 



^*Jc5'' ^ '*^ /^.\^/i) verse processes, at the 



i^% ^ ■* . ^ ^ ,^'^ tip of the urostyle, in the 



""^^^ (^ . ^' '^i^'^^S^-"" pubis of the pelvic gir- 

 ' ^ &^.) ^-'^^ '^^'^ ^ . die, in the hyoidand the 



® ^.'i^ i^^ '''*'f^ ^X^ <^'* cartilages of the larynx, 



/ " and at both ends of the 



^ sternum ; it forms the 



F(G. 33.-Cartilage from the head of the ^^^.^ ^^^^^ cranium and 

 femur, c, cells; c , cells m process of di- 

 vision ; c. s, empty cell space ; ;;/, matrix, the central axis of the 

 (After Parker and Parker.) lower iaw 



Calcified cartilage, which contains a deposit of lime salts 

 in the matrix, occurs in the suprascapula, the pelvis of old 

 frogs, and at the ends of some of the larger bones of the 

 limbs ; viz. the heads of the humerus and the femur. 



The structure of bone is similar to that of cartilage in that 

 it contains cells imbedded in a soHd matrix. In bone the 

 matrix is rendered firm by the deposit of carbonate and 

 phosphate of lime. By immersion in acid the lime salts may 

 be removed and a cartilaginous body having essentially the 

 same histological structure as bone remains. Bone, how- 

 ever, is not merely calcified cartilage ; it differs from it both 

 histologically and chemically. Cartilage is often the precur- 

 sor of bone, but in such cases the former is broken down 

 and bony tissue built up in its place. 



Two principal varieties of bone are usually distinguished, — 



