We have very little information about the nature of the forces which are concerned in such equilibria. They might be intermolecular forces o f the kind responsible for liquid crystal formation; probably diffusion forces are important, and several other suggestions are possible. But whatever their nature, they could only give rise to an equilibrium which is a morphological pattern i f in the first place they proceed from different points in the mass o f tissue. However far we can analyse the development o f a pattern, we shall therefore always be left with an initial heterogeneity to account for. The basis o f such a heterogeneity might be (i) local differences in chemical forces at different points on molecules formed by pattern-genes; (2) local differences in the cyto­ plasm o f the egg; (3) local differences between different parts o f the chromosomes. Differences o f these three kinds certainly exist. But it is difficult to see how local chemical differences within a molecule could give rise to sufficiently complicated patterns o f the right size. M ore­ over, there is no evidence that the linear arrangement o f genes has anything to do with the developmental patterns; in fact, the evidence o f translocations, inversions, etc., is directly against this. We are left with local differences o f the cytoplasm as the immediate source o f

1 General references: Henke 1933, I935-

the whole pattern o f the animal. During development new substances and tissues are produced by the interaction of different regions of the egg, and in this way the pattern gradually becomes more complex. I f the nature o f the reacting substances is altered by a gene substitution, the equilibrium which they attain will be altered and a new pattern produced.