THE AMEBIC A 



gray-red females the character will not always develop. 

 A typical count for such a cross was as follows: 



(The count of females is shown on the upper line, the 

 count of males on the lower.) A brother of the above 

 male, however, when similarly back-crossed, gave the fol- 

 lowing count : 



The sex-linked intensifier and the third chromosome in- 

 tensifier are inherited normally as before, for the females 

 have wings much more truncated than the males and the 

 reds are more truncated than the pinks. But, although 

 the truncate parent of this male contained gray in the 

 same chromosome as truncate, and the long-winged par- 

 ent contained black with long, all the truncate has 

 crossed over, away from the gray factor and into the 

 chromosome with black ! Not a single fly has the old com- 

 bination, gray truncate. It is next to impossible to im- 

 agine that the chromosomes of the second pair crossed 

 over in the synapsis period of all the spermatocytes, and 

 in all of them, between just these particular loci, when 

 normally there is no crossing-over at all in the male and 

 only 30 per cent, of crossing-over between these loci even 

 in the female. It is, therefore, necessary to conclude that 

 crossing-over took place once for all in a cell of the em- 

 bryo, and that, as usual, it did not occur at all during 

 spermatogenesis, although all the spermatocytes, of 

 course, inherited the cross-over combination. It is im- 

 possible to tell whether or not the chromosomes under- 

 went the regular process of synapsis at this early stage, 



