274 CALVIN B. BRIDGES 



If these were true crossovers, it is possible that their pro- 

 duction had no relation to the mechanism by which crossing 

 over is ordinarily effected. Thus, MuUer ('16) reported a case 

 of crossing over in the back-cross test of a certain Fi male from 

 the mating of truncate to black. However, all of the gametes of 

 this particular Fi male proved to be crossovers, so that crossing 

 over must have occurred, once for all, in an early cell of the 

 embryo, and, as usual, no crossing over whatever occurred during 

 spermatogenesis. The spermatozoa, all of which were descended 

 from this embryonic crossover cell, simply inherited the cross- 

 over combination. In the case of purple vestigial, a like ex- 

 planation would apply, except that in this case the crossing over 

 occurred in a somewhat later stage of the embryo, and in conse- 

 quence only a part of the spermatogonial cells carried the 

 crossover combination and only sperm decended from these 

 particular cells produced crossover progeny. 



That somatic crossing over has little analogy to the ordinary 

 type is proved by a similar case of embryonic crossing over in 

 the female, which was then followed by crossing over of the 

 ordinary type. A mating was made such that a certain class of 



daughters should all have the composition -, — , r- ?■ 



^ + + s g + 



Seven of the eight daughters tested had this expected compo- 

 sition, but one (no. 3464) gave only offspring corresponding to 



the composition -, — -. — . That is, the gene for lethal 9 



+ I9 s g + 



was found to be not in the chromosome in which it entered the 

 zygote, but in the homologous chromosome derived from the 

 other parent. As in the truncate X black case, this transmi- 

 gration took place after fertilization and so early in the em- 

 bryonic history that all the germ cells were descended from this 

 altered cell. A significant feature of this case is that while the 

 change must be described superficially as double crossing over, 



2 The symbols above the line represent the genes in the chromosome derived 

 from one of the two parents, those below from the other. The + signs represent 

 simply the wild-type allelomorphs of the mutants dealt with, and these signs 

 may usually be omitted. 



