BOXE. 



Fig. L. 



soft fibres or trabeculse which have already stretched across the interval ; and 

 the darkish granular opacity 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. Tins soft transparent matter, which 

 becomes ossified, may, wherever it occurs, be distinguished by the name of 

 " osteogenic substance," as proposed by H. Miiller, or simply of " osteogen." 

 It is or becomes fibrous in intimate structure, and for the most part finely 

 reticular, like the decalcified bone itself, but must not be confounded with 

 fibres which may pre-exist in the membranous tissue in which the bone is 

 growing. 



The granular corpuscles or cells everywhere cover in a dense layer the 

 osteogenic substance, and lie 

 in its meshes; most probably 

 they yield or excrete that 

 substance, and hence it has 

 been proposed to call them 

 " osteoblasts." On this view 

 the process might be com- 

 pared to the production, by 

 cells, of the matrix of carti- 

 lage and the intercellular 

 fibrillatiug substance ingrow- 

 ing connective tissue ; also 

 to the excretion of mem- 

 branous, cuticular, and other 

 deposits by cells, and layers 

 of cells, long since pointed 

 out by Kolliker. 



But some of the granular 

 cells are involved in the ossi- 

 fying matrix, and eventually 

 inclosed in lacunae. Single 

 cells may accordingly be seen 

 partially sunk in the recent 

 osteogenic deposit, which 

 then gradually grows over 

 them and buries them in its 

 substance ; and the cavity in 

 which the corpuscle is thus 

 inclosed becomes a lacuna. 



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

 gular network of soft and pellucid osteogenic sub- 

 stance, on which the calcification is encroaching ; 

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

 calcified at a and a' /, extremity formed of bundles 

 of soft osteogenic fibres. N.B. the structure repre- 

 sented was covered over and hidden by granular cor- 

 puscles which have been removed. In the calcified 

 part, a, b, c, superficial excavations are seen which 

 are probably commencing or incomplete lacunae, from 

 which the corpuscles have been washed out. From a 

 drawing by Mr. J. Marshall, F.R.S. 



Fig. L. THE GROWING END OP A SPICULUM FROM 

 THK PARIETAL BONE OP AN EMBRYO SHEEP at about 

 the same period of advancement as in Fig. XLIX. ; 

 magnified 150 diameters, but drawn under a power 

 of 350 diameters. 



Some observers state that,when 

 such a corpuscle is as yet but 

 half sunk in the growing sub- 

 stance, processes may be seen 

 passing from the imbedded side 

 into fine clefts of the matrix, 

 which close in around them and 

 become the canaliculi ; and that, 

 as the inclosure of the corpuscle 

 is completed, canaliculi are in 

 like manner formed in the rest 



of its circumference. It is also 



supposed that the canaliculi are afterwards extended by absorption, so as to anas- 

 tomose with those of neighbouring lacunse. But from all I can see of the process, 

 it seems more probable that, whilst the ossific matter closes in around the corpuscle 



