74 



FUNDAMENTALS OF CYTOLOGY 



divisions disturb the regularity of the pattern more than they do in an 

 epithehum, since the divisions may occur in so many different planes, but 

 through mutual adjustments of the plastic walls the average number of 

 sides continues to be close to 14 so long as the cells are of uniform size. 

 Cells at the surface of the mass have a smaller average number of sides. 

 Thd development of intercellular spaces in such tissues brings further 

 modifications of cell shape. 



Light on the problem of cell shape has also been sought through 

 experiments in which many balls of lead or putty were compressed in 

 cylinders. Although the resulting mass of polyhedrons did not originate 

 as a tissue does, it was found that when the balls were of uniform size 

 they came out of the press with an average of 1-i sides, with the exception 

 of the peripheral ones which had an average of 10.75. After mixtures 



Fig. 50. — n, transverse section of epidermal cells of cucumber, b, diagram of cell 

 division in a simple epithelium, c, model of an orthic tetrakaidecahedron. Explanation 

 in text. (After F. T. Lewis.) 



of large and small balls were compressed, the average number of sides 

 was more than 14 for the large ones and less than 14 for the small ones, 

 the average number for all taken together being close to 14. A method 

 has also been developed for making wax casts of cells for the study of 

 their shapes. 



This is but a brief glimpse of another field of cytological research that 

 should lead to a better understanding of the role of cells in growing and 

 differentiating tissues. One way in which a cell affects the behavior of 

 its neighbors is suggested by certain geometrical features mentioned 

 above: the division of a given cell adds one side to each of two others; 

 cells w^ith more than the average number of sides grow larger and divide 

 sooner than those with fewer sides. Hence cell division is not merely a 

 multiplication of units; it is also a factor in correlating the activities of 

 the cells. In Chap. II we stressed the point that the behaVior of regions 

 in a growing mass depends in part upon their positions in the whole. 

 We now see that cell partitions within a tissue affect the activities of the 

 units they separate not only by virtue of their constitution and consequent 

 effect upon th(> diffusion of materials, but also through the geometrical 

 form they impose upon the cells b.y tending to develop in accordance with 

 the laws of minimal surface area. 



