270 



BONE OR OSSEOUS TISSUE. 



fibres pari passu with their growth, and thus new bony spicules become continually 

 formed by calcification of the groups or bundles of osteogenic fibres. 



The earthy deposit occasionally appears in an isolated patch here and there on 

 some of the osteogenic fibres in advance of the main area of ossification (see 

 fig. 309, A, a). 



The osteogenic fibres become comparatively indistinct as they and the substance 

 between them calcifies ; they appear, however, to persist in the form of fine fibres, 

 such as are seen in the lamellae of the adult bone, although in the embryonic bone 

 their disposition is not lamellated, the bony matter having a somewhat coarsely 

 reticular structure. 



In this way the first bony matter becomes formed as a perforated plate or 

 network of osseous spicules, which, whilst becoming extended peripherally in the 

 way above described, gradu- 

 ally becomes thicker nearer 

 the centre, partly by the 

 deposit of bony matter upon 

 its surfaces, partly by the 

 projection from them of bony 

 spicules which are prolonged 

 like those at the periphery by 

 similar systems of osteogenic 



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Fig. 309. PART OF THE DEVELOPING PABIETAL BONE OF A POSTAL CAT (\\ INCH LONG). (From 

 drawings by Mr. J. Lawrence. ) (E. A. S. ) 



A, a piece of the growing edge slightly magnified, showing the bony spicules terminated by bunches 

 of osteogenic fibres ; a, an isolated bony spicule united to the main part of the ossification by a bundle of 

 osteogenic fibres. 



B, the part marked b of the smaller figure, highly magnified ; sp, bony spicules, with some of the 

 osteoblasts imbedded in them, producing the lacunae ; ost, osteoblasts partly imbedded in the newly 

 formed bone ; of, osteogenic fibres prolonging the spicules, with osteoblasts between them and applied to 

 them ; a, granules of calcareous deposit between the osteogenic fibres ; at b the granules have become 

 blended, and the matrix is clearer ; at c a continuity is established between the two adjacent spicules. 



fibres. The perforations in these first-formed bony plates correspond to the 

 bays which were seen between the advancing spicules, and to the meshes of the 

 bony network formed afterwards by the junction of the spicules, and as the bone 

 thickens they become enclosed and converted into reticulating interstices (like the 

 canals of a sponge), which are occupied by blood-vessels, and by the corpuscles 

 above mentioned. These corpuscles also everywhere cover the osteogenic fibres, to 

 which their flattened sides are often applied (fig. 309, B, ost). Where the osteogenic 

 fibres diverge from one another, the intervals are occupied by the same cells. It is 

 probable that the osteogenic substance is formed by the agency of the cells in 

 question, hence the name " osteoblasts " was assigned to them by Gegenbaur. 



