190 DISTRIBUTION OF THE CHROMOSOMES 



factors, since every spermatozoon receives, with the 

 above exceptions, all the chromosomes (paternal and 

 maternal) that the hybrid contains. On crossing the 

 hybrid to either parent, it is found that the offspring 

 actually are very much alike, i.e., have all received 

 practically the same factors — a striking contrast 

 to the result usually obtained in ''backcrosses." 

 In respect to just one character (a larval marking), 

 however, the above relation does not hold, but ordi- 

 nary Mendelian results are obtained, and this in turn 

 corresponds with the fact that a few chromosomes 

 do undergo segregation. In regard to the other char- 

 acters, not only are the offspring like each other, but 

 they resemble the hybrid more than either of the pure 

 species, corresponding with the fact that they contain 

 complete sets of chromosomes from both types. But 

 they do not look just hke the Fi hybrid, and cor- 

 respondingly one set of chromosomes is in the diploid, 

 the other in the haploid number. This is because they 

 receive a set of one species from both parents, but a 

 set of the other species only from the hybrid parent. 

 Federley also shows that when maturation takes place 

 in this triploid individual one set of chromosomes 

 does not undergo mating, but the others — presumably 

 those in the two identical sets— do pair with each 

 other, so that the total number is reduced to one bi- 

 valent set, and one single set. If the paired chromo- 

 somes separate and the unpaired ones divide, as oc- 

 curs in the Fi hybrid, the double number of chromo- 

 somes, a set of each species, will again be found in the 

 sperm, as was the case in the first hybrid. In other 



