This chapter (and the beginning of the next) possesses a curious blind spot: it is primarily concerned with the effect of surface-tension on the form of cells, yet it completely ignores all the beautiful experimental work, begun in the early 1930's by E. N. Harvey, K. C. Cole and others on the actual measurement of the surface-tension of cells. This is all excellently reviewed in a recent paper by Harvey; let me just state two of the important conclusions here. In the first place there are a number of clear demonstrations that the cell boundary primarily exerts membrane tension and not true surface-tension. In fact it is a composite of the two and the sum of all these tensions is referred to by Harvey as ‘tension at the surface’. The second point is that these tensions are extremely low, far too low to account by themselves for cell shape. Instead, then, we must look to the micro-structure of the cell periphery for an understanding of cell form. Despite the fact that no reference is made to this considerable body of experimental work, it must be admitted that D'Arcy Thompson seems, in certain passages, to be aware of the difficulties.

But this omission need not mar the value of the chapter if we think of D'Arcy Thompson's presentation as a model rather than a reality. The formula of Laplace remains a useful description of the sites of forces playing upon a cell, even though those forces are not surface-tension alone.