172 organisers: inducers of differentiation 



the blastomere CD, double monsters, each member of which 

 possesses a complete set of trunk organs, are produced. It might 

 therefore seem as though the somatoblasts (for the formation of 

 which the polar lobe or pole-plasms are essential) of Annelids and 

 Molluscs deserve the title of organiser.^ It is, however, unlikely 

 that these exert an effect similar to that of the amphibian dorsal 

 lip, or of an engrafted Planaria head : it is more probable that the 

 growth processes initiated by a single somatoblast automatically lead 

 to the production of a more or less complete set of trunk organs. 

 A hitherto unique type of determination is found in the wings of 

 moths. As is well known, in Lymantrta, intersexual types can be 

 produced by appropriate crosses of local races (see Chap, xii, p. 409). 

 The normal wing pigmentation is white in females, dark in males. 

 In male intersexes (i.e. animals which begin adult differentiation 

 as males but continue it as females) the wing shows a mosaic of 

 white (female) and dark (male) pigmentation. The quantity (total 

 area) of female-type pigmentation is directly proportional to the 

 degree of intersexuality as measured by other secondary sexual 

 characters, but the pattern is irregular and varies from specimen to 

 specimen. Careful observation shows that the limits of the male 

 and female areas are defined in reference to the course of the veins. 

 The appearance is as if there had been a flow of a certain quantity of 

 dark pigment through the veins. ^ However, from other work we 

 know that pigment deposition occurs in relation to the determina- 

 tion of the scales. The scales, if determined as female, develop 

 quickly ; if as male, develop slowly. The visible determination of 

 sexual type can be seen to occur long before the wings become pig- 

 mented. Meanwhile the processes leading to the deposition of 

 white pigment occur some time before the end of pupal develop- 

 ment, and those leading to the deposition of melanin occur later. 

 Pigment can only be deposited during a certain stage in the develop- 

 ment of a scale. Thus in the female the white pigment-precursors 

 find the female-determined scales at the right stage, while later, 

 when the processes leading to melanin-production occur, no scales 

 are available in which it can be deposited. The reverse is true in 

 males. The result is brought about by interaction of two indepen- 

 dent sets of processes. 



^ E. B. Wilson, 1929. ^ Goldschmidt, 1923, 1927. 



