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 



^s bones of the limbs, be- 



m. V yij'^^ tween the vertebrae and 



''!o rx ^ W**/^ at the ends of their trans- 



it M ^% L/ 



k)c5? *^ (^i.SifS) verse processes, at the 



^^ (5^. *^ Agj tip of the urostyle, in the 



t-'® ^(Q^ '^^'■'^. ., dle,inthehyoidandthe 



'0 ^«^ Q\ f® 09-^- c.*^ cartilages of the larynx, 



"'"* ~ / and at both ends of the 



^ sternum ; it forms the 

 FIG. 33._Cartilage from the head of the ^^^-^ ^^^^^ cranium and 

 femur. £, cells; c , cells in process of di- 

 vision ; t. .f, 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 soUd 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, — 



