CAKTILAGE. BONE. TEETH. 3 7 



The process of ossification from periosteum is less 

 complicated than that just described, by which car- 

 tilage is converted into bone. This fibro-vascular 

 membrane contains, as we have already stated, a large 

 number of little star-shaped or plasmatic cells. 

 When, by the addition of acetic acid, the ordinary 

 connective fibres of the membrane are made to dis- 

 appear, more than one layer, composed of elastic fibres 

 and branching cells, is brought in view, in which 

 both the form and arrangement of osseous cells are 

 faithfully represented. (PL VII. fig. 1, 3.) In the 

 deepest layers of the periosteum, where ossification 

 takes place, the plasmatic cells are seen to be more 

 numerous, and farther advanced in development than 

 elsewhere. The blastema in which they are imbedded 

 is also deeper in color, and this is explained by the 

 presence, already, of earthy salts. (PL VI. fig. V. 2, 

 3.) In fact the process of ossification is thus almost 

 perfected, for the plasmatic cell requires only to be 

 imbedded in and surrounded by earthy matter, to 

 become a cell of bone. However, the process is not 

 always effected in so simple a manner, for sometimes 

 the plasmatic cells do not present their usual star- 

 shaped prolongations, and in this case the filiform 

 processes, which ultimately form canaliculi, only make 

 their appearance whilst the incrustation of their cell- 

 walls is actually taking place. 



From our own investigations it is evident that the 

 ossification of the cranial bones is effected entirely by 

 the changes just delineated in their periosteal cover- 

 ings. (PL VII. fig. I.) The new deposits of bony 

 matter which take place in some grades- of perios- 



