INDIVIDUATION— FORMATION OF PATTERN AND SHAPE 421 



a more complicated way. For instance, each spot in a simultaneous rhyth- 

 mic pattern may have not a sharp boundary but one w^hich grades 

 gradually into the background; or there may be a reaction between the 

 spot and the background in such a way as to outhne its periphery. Again, 

 from a single spot or elongated area some substance may diffuse outwards 

 and give rise to a periodic pattern as a consequence of the well-known 

 (though little understood) Liesegang phenomenon. This form of rhythmic 

 pattern Henke refers to as a diffusion rhythm. Mere inspection is some- 

 times sufficient to suggest that a given rhythmic pattern belongs to 

 the simultaneous or the diffusion type. In the former one would expect the 

 pattern to be rather irregular but to show no indication of any change in 

 the wave-length, whereas in the latter one expects a more precise forma- 

 tion of the pattern and a gradual increase or decrease in wave-length. A 

 final distinction of the two types can, however, only be reached experi- 

 mentally. 



In a number of cases, indeed, experiment has demonstrated that diffu- 

 sion plays an important part in the production of a certain pattern. For 

 instance, in the wings shown in Fig. 20.3 the central field originates from 

 two pomts, one on the anterior margin of the wing and the other on the 

 posterior. From these, two streams of some substance spread across the 

 surface of the wing until they meet in the middle, as is shown by the fact 

 that if the wing is wounded by cauterisation during the period when the 



C37. ^:d. ck c» ^cm 



Figure 20.3 

 Diffusion as the process producing the Central Field in the wing of the meal- 

 worm Ephestia. The upper row show examples of inhibition of the diffusion 

 by cauterisations performed at the time when it would normally be pro- 

 ceeding (2- to 3-day pupa). In the middle row, the cauterisations were made 

 earlier; the diffusing material has merely by-passed the necrotic areas. In 

 the lowest row, a shows the normal extent of the central field; b a case 

 when It IS narrowed by the action of a gene 5y; c, a similar narrowing 

 caused by high-temperature treatment during the diffusion; d, a widened 

 tield produced by the gene Syb; e, a similar effect caused by early hich- 

 temperature treatment. (From Henke 1935, after Kiihn and v. Engelhardt ) 



