OSSIFICATION IX MEMBR.iXE. 



95 



integuments, the dura mater, and an intermediate membranous layer, 

 which differs from cartilage in its intimate structure as well as in its 

 more obvious ciiaracters, and in whicli the ossification proceeds. 



The commencing ossification of the parietal bone, which may be 

 selected as an example, appears to the naked eye in form of a network 

 in which the little bars or spicula of bone run in various directions, 

 and meet each other at short distances. By-and-by the ossified part, 

 becoming extended, gets thicker and closer in texture, especially towards 

 the centre, and the larger bony spicula which now appear, run out in 

 radiating lines to the cir- 

 cumference. The ossifica- 

 tion continues thus to 

 spread and consolidate un- 

 til the parietal meets the 

 neighbouring bones, with 

 wliich it is at length united 

 by a suture. 



The figure (54) repre- 

 sents the parietal bone of 

 an embryo sheep about two 

 inches and a half long, and 

 shows the character of the 

 ossification as it appears 

 when the object is mag- 

 nified about twelve dia- 

 meters. The bone is formed 

 in membrane as in the 

 human foetus, but a thin 

 plate of cartilage rises up 

 on its inside from the base 

 of the skull. The ossifica- 

 tion, however, is decided!}' 

 unconnected with the car- 

 tilage, and goes on in a 

 membrane lying outside of 

 it. 



When further examined 

 with a higher magnifying 

 power, the tissue or mem- 

 brane in which the ossifi- 

 cation is proceeding, ap- 

 ]iears to be made up of 

 fibres and granular cor- 

 puscles, with a soft amor- 

 phous or faintly granular uniting matter, and, in point of structure, might 

 not unaptly be compared to connective tissue in a certain stage of develop- 

 ment. The corpuscles are large, mostly two or three times the size of 

 blood-corpuscles ; their substance is granular in character, and, espe- 

 cially in specimens preserved in spirit, usually hides the nucleus. They 

 are densely pncked all over the area of ossification, covering the bony 

 spicula, and filling up their interstices ; so that, to bring the growing 

 parts into view, the corpuscles must be brushed away with a hair pencil, 

 or removed by short immersion of the specimen in weak solution of soda. 



Fig. 54. — Parietal Bone op an Embryo 

 Sheep. Size of the Embryo, 2^ inches. 



The small upper figure represents the bone of the 

 natural size, the larger figure is magnified about 12 

 diameters. The curved line, a, h, marks the height 

 to which the subjacent cartilaginous lamella ex- 

 tended. A few insulated particles of bone are seen 

 near the circumference, an ai)pearance which is quite 

 common at this stage. 



