556 THE FORMS OF TISSUES [ch. 



at last. Then each (provided they be of equal size) will be in touch 

 with twelve neighbours; and if the spheres were solid— were the 

 system not an emulsion but a " suspension "—the matter would end 

 here. But our hquid globules are capable of deformation, and the 

 points of contact are flattened in still closer packing into planes. 

 Ttey become polyhedral, and tend to take the form of rhombic 

 dodecahedra, or it may be even of 14-hedra, and the dispersion- 

 medium is reduced to mere films or pellicles between. At the stage 

 of mere twelve-point contact, the spherules constitute about 74 per 

 cent., and the disperse .medium 26 per cent., of the whole. But in 

 the final stage the phase-ratio has so altered that the disperse- 

 medium is but a small fraction of the whole, the thin film to which 

 it has been reduced has the appearance of a cell-membrane separating 

 the cells, and the microscopic structure of the whole corresponds to 

 the cellular configuration of a parenchymatous tissue*. 



Of certain groupings of cells 

 It follows from all that we have said that the problems connected 

 with the conformation of cells, and with the manner in which a 

 given space is partitioned by them, soon become complex; and 

 while this is so even when all our cells are equal and symmetrically 

 placed, it becomes vastly more so when cells varying even slightly 

 in size, in hardness, rigidity or other quahties, are packed together. 

 The mathematics of the case very soon become too hard for us, 

 but in its essence the phenomenon remains the same. We have 

 little reason to doubt, and no just cause to disbelieve, that the 

 whole configuration, for instance of an egg in the advanced stages 

 of segmentation, is accurately determined by simple physical laws: 

 just as much as in the early stages of two or four cells, during which 

 early stages we are able to recognise and demonstrate the forces 

 and their effects. But when mathematical investigation has become 

 too difficult, physical experiment can often reproduce the pheno- 

 mena which Nature exhibits, and which we are striving to com- 

 prehend. In an admirable research, M. Robert not only shewed 

 some years ago that the early segmentation of the egg of Trochus 

 (a marine univalve mollusc) proceeded in accordance with the laws 



* Cf. E. Hatschek, Homogeneous partitionings, etc., Phil. Mag. xxxiii, p. 83, 

 1917. 



