io6 COMPARATIVE ANATOMY 



corpuscles being homologous with ordinary connective tissue- 

 cells. In fact, true bone is formed in connective tissue, 

 whether it be what we call " cartilage bone," or whether it be 

 "membrane bone." It must be borne in mind that when we 

 speak of a " cartilage bone," such as the ex-occipital or pro- 

 otic of a frog's skull, we do not mean that the cartilages of the 

 cranium or of the otic capsule have been transformed into 

 bone. We only mean that the bone has been preceded by 

 cartilage, which latter has been absorbed and replaced by bone 

 in the manner to be described. There must be no confusion 

 between calcified cartilage and true bone. Cartilage is a non- 

 vascular tissue ; bone, with its system of blood-vessels running 

 in the Haversian canals, is highly vascular. Bone, as has been 

 said, may be developed directly from membrane, without being 

 preceded by cartilage. To take, as an example, the parietal 

 bone of a mammal, such as a sheep. Between the dura mater, 

 as the connective tissue layer surrounding the brain is called, 

 and the outer integument, is a layer of connective tissue, com- 

 posed of a matrix with fibres and numerous granular corpuscles. 

 It is in this membrane that each parietal bone appears in the 

 form of a number of spicules of bone radiating outwards from 

 a " centre of ossification," and joined together at intervals by 

 short, irregularly-placed tangential bars. The outer end of 

 each radial spicule is prolonged for some distance beyond the 

 actual calcareous spicule in the form of a bundle of parallel or 

 slightly divergent transparent fibres, the so-called osteogenic 

 fibres. They resemble white connective tissue fibres, except 

 that they are straighter, stiffer, and are less distinctly fibrillated. 

 The osteogenic fibres are covered by a layer of granular cells 

 called osteoblasts, applied to the surfaces of the fibres and 

 occupying their interstices when they diverge from one 

 another. The osteoblasts appear to form the osteogenic 

 fibres, and also to supply the calcareous deposit which is laid 

 down in the form of minute granules in the matrix between 

 the fibres. Eventually, by their coalescence, the granules form 

 the substance of the bone, the latter first appearing as a 

 network of spicules and splinters, but as ossification proceeds 

 the spaces between are gradually filled up by the continued 

 deposition of calcareous granules. In many cases, however, 

 the interstices are occupied by blood-vessels, and these are 

 surrounded by the advancing bone, and eventually form the 



