BIOLOGY. 377 



hybrids are sterile, but since first-generation hybrids are obtainable 

 it has been possible to test the allelomorphism of similar mutations. 

 To date about 25 mutant genes have been studied in D. simulans; 

 1 1 of these have actually been shown to be allelomorphic to pre\'iously 

 known mutant genes of D. melanog aster. Of these 7 are in the X-chro- 

 mosome in both species and show the same sequence. 



Another species, D. obscura, has been studied during the last two 

 years by D. E. Lancefield. He has obtamed upwards of 15 mutants. 

 The chief interest in this species is that it has a different chromosome 

 group from that of D. melanogaster. As C. W. Metz had sho\vn, 

 there are five pairs of chromosomes and the X-chromosome is nearly 

 twice as long as the X of melanogaster. It is significant, therefore, 

 to find that the chromosome map of the X is more than double the 

 length of that of melanogaster. A further point of importance is that 

 there is at least one mutant locus for every one of the chromosomes. 

 Provided no crossing-over takes place in the male (and such has been 

 found to be the case wherever tested), the number of "groups" corre- 

 sponds to the number of the chromosomes and is one greater than 

 that in D. melanogaster. 



One of the types of "minutes" referred to in our last report, now 

 called "diminished," has been found to be due to the loss of one of the 

 IV-chromosomes. The genetic evidence indicated that such an ex- 

 planation would cover the facts, and when the cells of diminished 

 females were studied, the absence of one small round chromosome was 

 observed. As a result of such a loss, recessive characters in the IVth 

 chromosome appear to be dominant in the Fi hybrid. A simple expla- 

 nation as to the origin of such a haplo-IVth individual is that it arose 

 through non-disjunction — that is, an egg had been formed io which both 

 IV-chromosomes had been eliminated in the polar body; or at a corre- 

 sponding di\ision of sperm-cell, one cell was left with no IV-chromosome. 



We have now also individuals with an additional IV-chromosome, 

 that is, with three IV-chromosomes. Such individuals shov/ many 

 characteristics that are the opposite of those shovv'n by individuals 

 that have only one IV-chromosome. The body-color is darker than 

 the wild type, while that of diminished is lighter than the wild type, 

 etc. \ATien triple-IV's are mated to eyeless (a recessive IV-chromo- 

 some character), and two Fi triplo-IV's are mated together, the eyeless 

 character reappears as 6 per cent of the eyeless progeny, instead of 

 the usual 25 per cent. If such an Fi triplo-IV is back-crossed to a 

 normal eyeless, then among the progeny 16 per cent instead of 50 

 per cent are eyeless. The fact that these individuals are triplo-IV's 

 has been demonstrated both by crosses to haplo-IV individuals and by 

 microscopic study of the chromosomes. 



Individuals with thi-ee X-chromosomes (which were formerly sup- 

 posed always to die) have recently been found, showing that they do 

 sometimes survive. They have certain structural peculiarities that 



