CHAPTER 9 



The Genetic Control of Pattern 



I . The Nature of Developmental Patterns^ 



Morphological patterns, such as we find in living organisms, are 

 arrangements of different substances or tissues in definite relative posi- 

 tions in space. These relative positions may not remain constant, but 

 may change as development proceeds, but the changes, if they occur, 

 follow a regular course. If we symbolize the pattern as it is at any 

 single instant by a point, the whole pattern as it changes would have 

 to be expressed as a line plotted against time. During development, 

 there is always some period when the tissue tends, after experimental 

 disturbance, to "regulate"; that is to say, the disturbed tissue gets back 

 to the normal course of the pattern-development and finally produces 

 a normal organ. The normal pattern-development is therefore, at least 

 during this regulatory period, an equilibrium state to which the mass 

 of tissue tends to return. 



We have very little information about the nature of the forces which 

 are concerned in such equilibria. They might be intermolecular forces 

 of 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 if in the first place they proceed from 

 different points in the mass of tissue. However far we can analyse the 

 development of a pattern, we shall therefore always be left with an 

 initial heterogeneity to account for. The basis of 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 of the egg; (3) local differences between different parts of the 

 chromosomes. Differences of 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 of the right size. More- 

 over, there is no evidence that the linear arrangement of genes has 

 anything to do with the developmental patterns; in fact, the evidence 

 of translocations, inversions, etc., is directly against this. We are left 

 with local differences of the cytoplasm as the immediate source of 

 ^ General references: Henke 1933, 1935. 



