PLATE 1 



EXPLANATION OF FIGURES 



Camera lucida drawings from entire bone which was fixed in 95 per cent alco- 

 hol and stained in gentian violet. The original magnification as indicated for 

 each figure was reduced in reproduction two fifths. 



1 Relationship of cells in the ethmoid bone of an adult dog. The proto- 

 plasmic processes show an unusual degree of anastomosis. This is partly due 

 to the fact that the anastomosis is indicated as seen under a slightly shifting 

 focus. X 567. 



2 Group of young cells near the periphery of parietal bone from a 15-day old 

 rat. Cells arranged in rows and show signs of rapid division. Some nuclei are 

 still in the process of division. X 720. 



3 Group of cells from the center of a 15 day old rat parietal. Centrosomes 

 can be seen in a number of these cells. X 720. 



4 Three cells from the center of parietal bone in a one day old rat. The 

 striking features of this picture are the large protoplasmic masses almost sepa- 

 rated from the main cell bodies. The mass to the left contains nuclear fragments. 

 X 720. 



5 Cells from the parietal of a new born rat. Note broad protoplasmic bands 

 connecting the different cells. X 720. 



6 Cells from the peripheral portion of a 15 day old rat parietal. These cells 

 are younger than those in figure 2. They have small protoplasmic processes 

 and resemble osteoblasts. The matrix is slightly tinted by the stain which 

 indicates that not much bone is deposited. Several nuclei are already dividing. 

 X 7.20. 



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