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GENETICS AND EUGENICS 



for on this hypothesis, but many other cases in Drosophila 

 cannot, for which reason Morgan and Bridges favor a differ- 

 ent explanation. B and C (Fig. 154) are explanations of sex- 

 mosaics offered at different times by Morgan. In B it is 

 supposed that two sperms have entered the egg, one of 

 which united with the egg-nucleus and produced a female 

 (2X), hybrid as to sex-linked characters, the other developing 

 by itself produced male parts showing only characters of the 

 father. This explanation evidently will not fit the case of 

 (Fig. 153) because the male side of the fly inherits from the 



Fig. 154. Three different explanations which have been offered to account for the production of gynan- 

 dromorphs (sex-mosaics or scx-intergrades) in Drosophila. See text. (After Morgan.) 



mother, not the father. An alternative explanation, C, is 

 offered by Morgan for such cases as this. It is supposed that 

 the egg has been normally fertilized but that in a division 

 of the fertilized nucleus, one division product of an X- 

 chromosome gets left behind at the middle of the spindle. 

 Thus one daughter nucleus gets two X-chromosomes (fe- 

 male) and the other only one (male). Whether the male 

 part shows maternal or paternal characters will depend on 

 which X-chromosome (maternal or paternal) was eliminated. 

 Explanation C is thus an alternative to A for cases in which 

 the male part of the mosaic shows maternal characters, and it 

 also affords an explanation (alternative to B) of cases in which 

 the male part of the mosaic shows paternal characters. 

 In contrast to the case of insects, the dependence of second- 



