SEGREGATION OF HOMOLOGOUS CHROMOSOMES 489 



and cytology in his study of the moth, Pygaera. He crossed 

 P. anachoreta and P. curtula, and found by a study of the 

 spermatogenesis of the hybrid, that pairing occurs between 

 only two chromosomes; thus in the first spermatocyte of the 

 parent forms there are twenty-nine or thirty chromosomes, 

 while in those of the hybrid there are forty-eight of about one- 

 half the size of the parental forms. When a back cross was 

 made with one of the parent species, normal pairing took place 

 between about one-half of the chromosomes of the hybrid and 

 those of the parent species to which the cross was made, giving 

 about thirty large paired chromosomes among a corresponding 

 number of small unpaired ones. As one would expect from this 

 cytological knowledge, both the primary hybrids and those 

 resulting from the back cross were intermediate in all characters 

 except one or two which showed normal dominance and segre- 

 gation. The difficulty in working with such a form lies in the 

 large number of chromosomes and in the lack of any means of 

 distinguishing between them. 



The work of Morgan and his students on Drosophila ampelo- 

 phila is too well known to need much discussion. Here there 

 is the advantage of a small number of chromosomes which 

 differ in size and shape. According to Metz ('14) there are four 

 pairs as follows: a pair of microchromosomes, a pair of sex 

 chromosomes, equal in the female (XX), unequal in the male 

 (XT), and two large V-shaped pairs. 



Breeding tests with numerous mutants have shown one large 

 group of genes, over thirty in number, to be sex-linked, hence 

 borne, presumably, in the X chromosome. Two other great 

 groups of over twenty members each, segregate independently 

 and are assumed to correspond to the two large euchromosom.es. 

 Two characters so far- — bent wings reported by Muller ('14) and 

 eyeless, by Hoge ('15) — have been found whose genes segregate 

 independently of the other three groups. Should mutations 

 occur with equal freedom at any locus in the chromatin it would 

 be expected, as pointed out by Muller, that the small microchro- 

 mosomes would show fewer mutations than the larger chromo- 

 somes, and since the genes for bent wings and eyeless must lie 



