THE SHAPE OF CELLS. 551 



The relative levels of the upper surfaces of all the actual cells shown in 

 Figure 21 are like those in the pattern, Figure 20. Eighteen of the 

 surfaces appearing in Figure 21 have been marked with figures indi- 

 cating the number of their sides, and they correspond, both in respect 

 to these numbers and in their relations to one another, with the 

 eighteen surfaces similarly marked in the group of orthic tetrakaide- 

 cahedra. 



The problem which it was undertaken to solve, by presenting 

 actual examples for inspection, is that of the shape of cells when sur- 

 rounded on all sides and compressed by similar cells. Elder-pith was 

 selected as presumably typical of this condition, and so far as elder- 

 pith is concerned, the cells are shown to be tetrakaidecahedra, modi- 

 fied, especially through cell-division, in rather definite ways. The 

 surprisingly simple manner of restoring the tetrakaidecahedral form 

 after the transverse division of an entire group of cells, was brought 

 to light by the direct observation of the models. It is a pleasure to 

 acknowledge the way in which the mathematicians and physicists 

 have anticipated our principal conclusion; yet in - such shapes as we 

 have found, they may not recognize the verification of their calcula- 

 tions and experiments. 



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 Buffon, M. de. Histoire naturelle. Nouvelle edition. T. 5. Paris, 1769. 



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 Errera, Leo. Sur une condition fondamentale d'equilibre des cellules 



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