194 



AN INTRODUCTION TO MODERN GENETICS 



Lepidopteran wing has been worked out by Kiihn^ and his students in 

 the meahnoth Ephestia kuhniella. The essentials of the pattern on the 

 forewing are a central mid-field which is bounded by two bands, each 

 consisting of a white stripe between two dark stripes, while proximal 

 to this there is a basal field and distal to it an outer field. The mid-field 

 is an element of the pattern which is said to belong to the "symmetry 

 system/' and a similar element can be recognized in many Lepidop- 

 teran patterns. 

 The mid-field is produced by two streams of some agency, probably 



Fig. 90. Diffusion and Pattern. — A diagram of a case in which the pattern is 

 determined by the diffusion of pigment from two bands crossing a Lepidopteran 

 wing. Diffusion begins when the gene-controlled processes (indicated by the lines 

 M66, 0066, and aabh) produce a threshold concentration Y of pigment or pre- 

 cursor, and continues until some definite stage of development X. The extent 

 of the dark bands depends on the length of time during which diffusion of pigment 

 occurs, and that again depends on the rates of the reactions M66, 00B6, and aabb. 



(Modified from Goldschmidt.) 



of some substance, which start from the anterior and posterior edges of 

 the wing and then spread out from the middle towards the sides. The 

 stream is proceeding during the period between the 24th and 72nd 

 hour after pupation, at 18° C. If the pupal wing is wounded during 

 this period by burning with a cautery, the stream appears to be frozen 

 in the position which it has reached when the burn is made. By burning 

 at different ages, a series of pictures of the stream can be obtained. If 

 the burn is made earlier, the stream proceeds to completion, but is 

 checked by the dead tissue. 



We are still ignorant of the nature of the streaming agent or sub- 

 stance, and even do not know exactly what it does. The first effect 

 which can be detected is an alteration in the growth rate (measured by 



^ Ktihii 1932, 1936. 



