THE DEVELOPMENT OF THE TEETH. 169 



centre of the osteoblast cell as a ' space,' of course it is not 

 hollow, but consists of uncalcified matrix, and in this situa- 

 tion lies the nucleus of the cell. 



In carmine-stained preparations from the teeth of calves 

 a round nucleus may sometimes be seen lying in the stellate 

 " lacuna ; " the nucleus soon disappears, and plays no active 

 part in determining the form of the lacuna. The nucleus 

 may also be seen in the developing bones of human foetuses 

 and, though this is difficult to understand, the traces of the 

 nucleus seem to be beautifully preserved in the lacunse of 

 a supposed Pterodactyle bone from the Wealden, a section 

 from which was figured by my father in the paper referred 

 to. Exactly as calcification, advancing with irregularity in 



FIG. 76 (M. 



the interior of an individual cell, fails to render it homo 

 geneous by pervading its whole substance, so it may fail so 

 completely to unite contiguous cells as to obliterate their 

 contours. A lacuna, surrounded by such a contour line, 

 mapping the limits of the original cell, or cluster of cells, is 

 what is termed an " encapsuled lacuna." 



That which determines the formation of a lacuna, or an 

 encapsuled lacuna, at any particular spot, is unknown : all 

 that can certainly be said upon the subject is embodied in 



(') Encapsuled lacunae 



