420 



PRINCIPLES OF EMBRYOLOGY 



instance, by the formation of spots of more or less similar size lying at 

 roughly equal distances from one another, or a series of Hnes at approxi- 

 mately equal distances apart. These he speaks of as 'simultaneous rhythms', 

 distinguishing them in this way from other rhythmic patterns which 

 involve time, which v^dll be mentioned later. The simultaneous rhythm 

 may be on a small scale in relation to the whole area covered, in which 



m;£ 



Figure 20.2 



Types of pattern, according to Henke, illustrated in lepidopteran wings: 

 a, spatter pattern; h, c, d, e, 'simultaneous rhythms' of increasing wavelength, 

 leading to the case/ when there is only a single element present; ^, a single 

 spot which has a transitional zone around it; ^, a diffusion field; i, accumula- 

 tion of diffusing material at the boundar)'; k, I, m, are other examples of 

 boundary accumulations in more complex patterns; n, a 'centric rhythm' 

 resulting from a Liesegang-like phenomenon in a diffusion field; 0, p, q, 

 successive diffusion fields ; r, s, interactions of the diffusing substance with 

 the surroundings. (After Henke 1948). 



case the area will include a number of elements of the pattern. Alterna- 

 tively, if the periodicity is on a larger scale, there may be few, or in an 

 extreme case only one, repetition of the basic element. 



Henke seems to irrtply that such rhythmic or periodic patterns can 

 arise spontaneously within an originally uniform region. He suggests 

 (1948) that the periodicity arises through some form of competition, pre- 

 sumably for diffusable substrate materials, between spots which were 

 originally irregular in size and in disposition. A more precise account of 

 how such regular patterns can arise has been provided by Turing (1952). 

 We shall return to this later. 



Henke then considers a number of other types of pattern which arise in 



