4l8 PRINCIPLES OF EMBRYOLOGY 



according to the quantity of hormone injected and the rate of its ex- 

 cretion, quite different visible patterns might result; but they are all 

 secondary consequences of the thresholds and time-lags which vary in a 

 defmite manner within the feather. It is the arrangement of these varia- 

 tions which must be regarded as the primary pattern. It is comparatively 

 easy to understand how, once the primary pattern is given, various mani- 

 festations of it can be made visible by treatments such as hormone 

 injections. The fundamental problem is to understand how the time-lags, 

 thresholds, etc. come to be arranged in a pattern in the first place. (It 

 should be mentioned that the 'classical' story of hormone-induced feather 

 patterns, given above, has been severely criticised by 'Espinasse [1939]. It 

 has been quoted here because it brings out very clearly the distinction 

 between a primary pattern and the derived expressions of it.) 



2. The origination of pattern 



One of the most thorough investigations of formation of primary 

 patterns has been concerned with the coloration of the wings of Lepidop- 

 tera (Reviews: Henke, 1935, 1948). The comparative study of nearly 

 related species has made it possible to distinguish a number of different 

 elements, or systems of elements, which are combined together to form 

 the pattern of any particular wing. In the most general form there are 

 three such systems, which in Henke's terminology are referred to as 

 'fields' ; first, a general field, which can be thought of as covering the 

 whole wing; second, enclosed within this is a peripheral field; and third, 

 enclosed within that again, a central field. Both the central field and the 

 peripheral field usually stretch right across the wing from its anterior to 

 its posterior margin so that the general field occurs only at the base and 

 at the lateral edge. There are usually strongly marked features at the 

 boundaries between the fields forming a series of lines (the 'Querbinden' 

 or transverse bands) (Fig. 20.1). 



These three main fields seem to be epigenetically more or less indepen- 

 dent. They can be very differently developed in different species. Quite 

 often the peripheral field extends right down to the base of the wing, so 

 that the general field disappears in this region and occurs only at the distal 

 edge of the wing. Moreover, experiment shows that different fields are 

 determined at different times during the development of the wing-bud. 

 If small wounds are made by cauterisation in the developing bud after a 

 given element in the pattern is determined, the resulting wing will show 

 the normal pattern disturbed only by the presence of the dead area 

 (Fig. 20.1). If, on the other hand, at the time of operation the pattern 

 was not fully determined, its development will be modified in some way 



