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 lacune. 
OOO 
Fic. 36.—Diagram of growth of bone. A, from an animal recently fed with madder 
causing a layer of bong (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 vertebre. 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 
