INDIVIDUATION— FORMATION OF PATTERN AND SHAPE 419 



(as we shall see later, this modification may take the form of an arrest of 

 the pattern at an early stage in its development). If cauterisations are made 

 at different stages it is found that some elements of the pattern may act 

 as though they were fuUy determined at a time at which others are stiU 

 capable of bemg modified. We have therefore to consider each of the 

 fields as representing an independent unit within which pattern formation 

 is proceeding. This means that the lepidopteran wings are rather compli- 



Ptriphtral ,.' ,^"^ 



field y' I ^^nlraX 



y I held 



Figure 20.1 

 On left: generalised scheme for the three main elements in the wing pattern 

 in butterflies G/indicates the maximum extent of the General Field P/" of 

 the Peripheral Field (= 'Umfeld'), and C/of the Central Field ('Zentralfeld') 

 On right: the results of cauterising the pupal wing at various stages. In the 

 graph, age at operation is given (in days after pupation) as abscissa ; as ordin- 

 ate IS shown the percentage of cases in which the Peripheral Field or the 

 Central Field behave in a mosaic manner which shows that they have already 

 been determmed. The three wings drawn on the graph illustrate the results 

 ot an early operation, at a time when only the spots of the edge (General 

 Field) are determmed then a case in which the Peripheral Field is determin- 

 ed but the Central Field not, and finally a case in which the whole pattern is 

 determined. (After Henke 1948.) 



cated examples of pattern, since they consist of a number of different 

 mdependent areas rather than a single one; but this complexity is com- 

 pensated for by the great variety of patterns which are available in differ- 

 ent species and the ease with which they can be studied. 



Henke has discussed the different types of pattern which might theore- 

 tically be expected to form within any one area (Fig. 20.2). Before any 

 pattern appears one must imagine that the area is more or less homogen- 

 eous. The simplest pattern would be a random distribution of spots of 

 various sizes, the frequency of the different sizes falling into a normal 

 distribution. This he caUs a 'spatter' pattern. The next class he distinguishes 

 IS that m which some sign of periodicity or rhythm can be seen- for 



