SKELETON. 43 



and those passing through a cartilage stage (cartilage bones) can only 

 be recognized by following the ontogeny of the element in question. 



As stated above, there is much evidence to show that the membrane bones are 

 dermal bones which have sunk to a deeper position and have become secondarily 

 associated with the endoskeleton. This is especially evident in the skulls of some 

 of the lower ganoids. Ossification of cartilage takes place in two ways. In 

 ectochondrostosis the deposit of lime salts begins on the deeper surface of the 

 perichondrium and gradually invades the cartilage. In entochondrostosis the 

 cartilage becomes broken down in the interior, some of the cells becoming modified 

 into osteoblasts, and from these as centres of ossification, the process of bone forma- 

 tion extends in all directions. In ectochondrostosis at least, the centres of ossifica- 

 tion may have been derived, phylogenetically, from elements of the dermal skeleton. 



In ossification the bone is developed in layers, between which the osteoblasts are 

 arranged. In the elasmobranchs the skeleton is frequently strengthened by 

 deposits of lime, but this calcified cartilage differs from bone in that the deposits 

 of lime take the form of polygonal plates and there are no lacunae. 



FIG. 36. Diagram of growth of bone. A, from an animal recently fed with madder 

 causing a layer of bone (black) colored by the dye; B, later, no madder fed for some time, 

 a deposit of colorless bone on outside of colored layer, internal layer thinner; C, still later, 

 outer layer thicker, inner layer absorbed. 



Many bones increase in length by the addition of epiphyses at the ends. These 

 are separate ossifications which only unite with the main bone at the time the adult 

 condition is reached. The increase in diameter has some interesting features. In 

 animals fed with madder, the bone formed during the feeding is colored. In this 

 way it is found that the new bone (fig. 36, A) is laid down on the outside of the 

 other, and that with growth (B and C), the 'marrow cavity' on the inside is in- 

 creased in size by the resorption of the bone already formed. 



For convenience of treatment the endoskeleton is divided into axial 

 and appendicular portions, the axial consisting of the vertebral column 

 (backbone) and the skull, together with the ribs and sternum which are 

 closely associated with the vertebrae. The appendicular skeleton in- 

 cludes the framework of the limbs and fins and the girdles to which 

 they may be attached. 



Axial Skeleton. 



Both the skull and the vertebral column surround and protect the 

 brain and spinal cord, and in this way the skull is an enlarged and 



