EOXE. 



On observins; more closely the points of the gTOwinc^ osseous rays at 

 the circnmferencc of the bone, where they shoot out into the soft tissue, 

 it will be seen that the portion of them already calcifiecl is granular and 

 rather dark in appearance (fig. 55, a, h, c), but that this character is 

 gradually lost as they are traced further outwards in the membrane, in 

 Avhich they are prolonged for a little way in form of soft and pliant 

 bundles of transparent fibres (fig. 55,/). Further inwards, where the 

 slender rods or bars of bone are already in great part hard, their calcified 



substance is coated over 

 (although unequally) with 

 transparent and as yet soft 

 and imperfectly calcified 

 matter, by which they grow 

 in thickness ; and this os- 

 sifying substance spreads 

 out at their sides, and en- 

 croaches on the interven- 

 ing space, in form of a 

 bright trellis-work (fig. 55, 

 d), thin towards its outer 

 limit, and there composed 

 of fine fasciculi, but denser 

 and closer nearer the bone, 

 where the trabeculce are 

 thick and round, and 

 already granular from com- 

 mencing' earthy impregna- 

 tion. The interstices of 

 this mesh-work are in some 

 parts occupied by one or 

 more of the corpuscles, but 

 at other parts they are re- 

 duced to short narrow clefts 

 or mere pores. The ap- 

 pearance here described is 

 especially well seen at those 

 places where a cross bridge 

 of bone is being formed 

 between two long spicula 

 (as at e) ; we may there 

 distinguish the clear soft 

 fibres or trabecule which 

 have already stretched 

 across the interval, and 

 the darkish granular opa- 

 city indicating the earthy 

 deposit (a, a') may be per- 

 ceived advancing- into them and shading off gradually into their 

 pellucid substance without a precise limit. This soft transparent 

 matter, which becomes ossified, may, wherever it occurs, be distin- 

 guished by the name of " osteogenic substance," as proposed by H. 

 Miiller, or simply of " osteogen." It is or becomes fibrous in intimate 

 structure, and ibr the most part finely reticular, like the decalcified 



Growing E:^d of a Spicclum from 

 THE Parietal Bone of an Embryo Sheep at 

 about the same period of advancement as in Fig. 

 54 ; magnified 150 diameters, but dra-«Ti under 

 a power of 350 diameters. 



rt, b, c, and «', parts already calcified ; d, d, irre- 

 gular network of soft and pellucid osteogenic sub- 

 stance, on which the calcification is encroaching; 

 a, e, «', a connecting bar or bridge still soft at e, but 

 calcified at a and cil ; f, extremity formed of bundles 

 of soft osteogenic fibres. The structure repre- 

 sented was covered over and hidden by granular cor- 

 puscles, or osteoblasts, which have been removed. 

 In the calcified part, a, h, c, superficial excavations 

 are seen which ai-e probably commencing or incom- 

 plete lacunae, from which the corpuscles have been 

 washed out. From a drawing by Prof. J. Marshall, 

 F.R.S. 



