DEVELOPMENT OF BONE. 79 



lage-cells disintegrate and disappear, and that the 

 osteoblasts are produced from cells which accom- 

 pany the blood-vessels. Still others regard them as 

 white blood-cells, modified and endowed with new 

 functional powers. In the intra-membranous and 

 sub-periosteal ossification, many suppose that they 

 are formed from connective-tissue cells. So little is 

 absolutely known, however, as to their genesis, that 

 while recognizing their importance in bone-forma- 

 tion, we can regard none of these various theories as 

 to their origin as definitely established. 



TECHNIQUE. 



Intra-cartilaginous and Sub-periosteal Ossification. A 

 long bone, from a nearly mature foetus or a young ani- 

 mal, should be carefully removed without injuring the 

 periosteum, and decalcified. After imbedding in celloi- 

 din, thin longitudinal sections are made with a micro- 

 tome through the ossification zone, embracing the tissue 

 for a considerable distance on either side of it. The 

 sections are stained double and mounted in balsam. 



Intra-membranous Ossification. To study the early 

 stages of this process, a young embryo (if from the sheep 

 or pig, four to six cms. long) should be soaked for a few 

 days in Miiller's fluid, and a bit corresponding to the por- 

 tion of one of the parietal bones cut out, and the skin, 

 muscles, and dura mater torn away with forceps under 

 water. The membrane in which ossification is occurring 

 is now to be carefully brushed with a stiff pencil until 



