300 J- T. PATTERSON. 



such as that of Drosophila. He says: "If the same sort of 

 primary non-disjunction which has been assumed to give rise 

 to XX'X' cells in an XX female, should take place at a cleavage 

 stage, gynandromorphs and mosaics would result. One might 

 expect at an early cleavage division, particularly the first, a 

 relatively large number of X-XX'X' divisions, for the greatly 

 condensed chromosomes introduced by the sperm do not for 

 some time attain the state or the appearance of those of the 

 egg nucleus. If the paternal X of a female were slower than the 

 maternal in preparing for division, it might lag upon the spindle 

 so that both daughter X's would become included in the same 

 cell. The portion of the fly which comes from the X cell should 

 be male and should show the sex-linked characters of the mother. 

 Such a process may be the explanation of the large number of 

 lateral gynandromorphs of Drosophila. When the X-XX'X' 

 division occurs at a later cleavage stage we may have mosaics, 

 as for example, a red female with a patch of white facets in 

 the eye." 1 



In non-disjunction the per cent, of males in any brood would 

 be determined, as in disjunction, by the time at which the 

 division takes place. However, there would be this difference, 

 that should the first cleavage be non-disjunctional, the expected 

 percentage of males would be fifty instead of one hundred ; should 

 the second cleavage be non-disjunctional, twenty-five per cent, 

 would be males instead of fifty, and so on, decreasingly through- 

 out the series of cleavage divisions. 



The third possibility mentioned above is in reality only a 

 modified form of non-disjunction, and the percentage of males 

 would be determined in accordance with the same law. 



The fourth possibility suggested above is of interest because 

 such a division would result in the production of blastomeres 

 devoid of sex chromosomes. Cells which contain no X chromo- 

 somes may be responsible for the so-called asexial larvae that 

 develop from the egg of this species. We shall consider this 

 point further in connection with a discussion of these peculiar 

 larvae. 



1 Loc. cit., p. 136. 



