INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 417 



by differential imbibition of water, by reaction to pH gradients, by 

 differential adhesion to other neighbouring cell masses, by the move- 

 ments of its individual constituent cells, and probably in many other ways. 

 It seems impossible to hope that we shall ever discover any single basic 

 mechanism of pattern formation or morphogenesis, as we may still hope 

 to fmd, for instance in the mechanism of protein synthesis and its control 

 by genes, the fundamental mechanism for substantive differentiation. In 

 discussing pattern formation and morphogenesis, therefore, one can 

 hardly hope to do more than provide a number of illustrations of the 

 general nature of the processes which are at work. 



I. Primary and secondary expressions of pattern 



Many of the most striking animal patterns which we can observe are 

 probably secondary or derived expressions of the underlying primary 

 pattern, and it is the formation of the latter rather than that of the visible 

 configuration derived from it which presents the really interesting prob- 

 lem. An example will make the distinction clear. If certain hormones 

 (thyroxin or oestrone) are injected into fowls of certain breeds a change 

 occurs in the colour of those parts of the feathers which are being formed 

 while the hormone content of the blood is at a high level. Lillie and 

 Juhn (1932) studied the shape of the coloured region which is produced 

 in response to single doses of various sizes. They showed that the tlireshold 

 of hormone concentration, which has to be surpassed before a colour 

 alteration is produced, is lowest near the rachis of the feather, and rises 

 towards the sides. They also came to the conclusion that the various parts 

 of the feather differ in the time-lag which has to elapse between the 

 attaiimient of the hormone threshold and the actual deposition of altered 

 pigment. 



On the basis of these two variables it is easy to see that one might obtain 

 a pattern consisting of a single spot near the rachis, by making an injection 

 which did not raise the concentration as high as the threshold of the lateral 

 parts of the feather. If, however, the injected dose were larger, so that it 

 surpassed the threshold even of the lateral parts, some form of transverse 

 bar would be obtained. The shape of this bar would depend on the rela- 

 tion between the time taken for the hormone level to fall again by excre- 

 tion and the time-lags of the various parts of the feather. Indeed, if the 

 hormone were excreted very rapidly the central parts of the feather, with 

 their long time-lag, might fail to respond at all, although the concentra- 

 tion had for a short time reached the necessary threshold and had produced 

 an effect on the more quickly-reacting lateral parts of the feather. This 

 would give rise to a pattern consisting of two spots near the edges. Thus 



