144 T. H. BAST 



cells of a one day old bone (fig. 3) are 16 micra apart while 

 those more peripherally located are separated by only 4 micra, 

 (figs. 2 and 6). Figm'es 2, 3, and 6 are drawings from the parietal 

 bone of a rat fifteen days old. 



A series of young rats, up to three and one-half weeks old 

 including the series already mentioned, bear out these observa- 

 tions. The younger cells as a rule are large and close together 

 while the older cells are small and far apart. 



6. Relation of cells to lacunae 



In all of the preparations, fixed in alcohol and carefully passed 

 through the steps of the technique outlined above, the cells 

 completely fill the lacunae. In preparations which were care- 

 lessly handled or allowed to become dry at any time the cells 

 were retracted from the bony wall. It appears therefore that 

 the term lacunae has no significance other than a place occupied 

 by a tissue cell. Thus in any tissue when a cell is removed a 

 pocket is left which might be called a lacunae with the same 

 degree of correctness as in bone except that due to the plasticity 

 of most tissues the space would collapse. 



c. Protoplasmic processes of bone cells 



Not only do the cell bodies completely fill the so-called lacunae 

 but the cell processes fill the so-called canaliculi also. This 

 is especially apparent in figures 2, 5 and 6. In older bones these 

 communicating processes are much finer but there is no indica- 

 tion that they are retracted. In young cells the processes are 

 short and broad at the base and the protoplasmic continuity 

 is unmistakable. The protoplasmic continuity and ^anastomo- 

 sis is also shown in older bone (figs. 3 and 4). These figures 

 further show large protoplasmic masses destitute of a nucleus, 

 connected to the cell body by a narrow process. In older bone 

 such masses are rare; but in their places one finds slight enlarge- 

 ments at the points where the anastomosis of several processes 

 occur (figs. 1 and 3). Figure 1, which is a drawing from the 



