166 



The Nucleus and Cytoplasm in Development 



in both male and female anlagen but a 

 response by formation of a sex-comb only 

 of cells with the male genotype. An ex- 

 perimental embryologist would attack the 

 problem by transplantation of male or fe- 

 male embryonic tissue into the prospective 

 sex-comb region of female or male embry- 

 onic discs. The geneticists may accomplish 

 equivalent results by studying gynandro- 

 morphs in which, early in development, 

 genetically male patches of tissue of varying 



A B C 



Fig. 44. First tarsal joint of Drosophila melano- 

 gaster. A, Normal male; distal (upper right) sex- 

 comb consisting of a row of ten teeth. B, Gynander; 

 sex-comb consisting of four teeth, interrupted by a 

 bristle of female genotype. C, Gynander; sex-comb 

 consisting of two teeth located in a region normally 

 not occupied by sex-comb structures. (After Stern 

 and Hannah, '50.) 



extent may originate in the appropriate re- 

 gions of otherwise genetically female tissues 

 (Stern and Hannah, '50). The observations 

 indicate that male and female genotypes 

 alike lead to the emergence of a sex-comb 

 field but that the response of the cells in the 

 region of the prospective cells depends on 

 their own genotype, male cells forming teeth 

 of the sex-comb even if surrounded by fe- 

 male tissvie and female cells being unable to 

 form teeth even if in the appropriate region 

 of a preponderantly male disc (Fig. 445, C). 

 In a secondary fashion, however, the dif- 

 ferent genotypes affect the field itself. When 

 the cells of the typical sex-comb region are 

 female in constitution and therefore unable 

 to form a sex-comb, cells of male constitu- 

 tion near by may differentiate into teeth of 

 a sex-comb even though they are in a region 

 which normally lies outside of the sex-comb 

 region. It seems that the development of the 



sex-comb in the typical region modifies the 

 surroundings so that no differentiation into 

 further teeth occurs. Absence of tooth forma- 

 tion in the typical region results in a modi- 

 fication of the extent or intensity of the out- 

 lying regions of the field such that male cells 

 can respond to it which would not have done 

 otherwise. This analysis of the origin of the 

 sex-comb may serve as an illustration for 

 many other cases where specific genotypes 

 change the course of development both by 

 varying the response of localized regions and 

 by remolding more general developmental 

 configurations. 



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