124 bulletin: museum of comparative zoology. 



Drosophila, it must be admitted that they go very far towards estab- 

 lishing a direct relationship between the elii'omosomes and the trans- 

 mission of Mendelian characters. Perhaps the most convincing evi- 

 dence of this kind is that obtained b}^ Bridges. He has found that in 

 certain strains involving sex-linked inheritance, some exceptional 

 females appeared which were like their mothers in every respect, and 

 showed no transmission of sex-linked characters from the father, 

 although such transmission would be expected, since the male sex 

 formula is XY and that of the female is XX. Furthermore, he found 

 that such exceptionally produced females inherit directly from their 

 mother the power of producing like exceptions (about 5%). The 

 explanation advanced by Bridges ('14) was that "the sex-linked genes 

 were borne by the X-chromosomes and that 10% of the eggs of the 

 exceptional females retained both of the Z^-chromosomes, or conversely 

 lost both to the polar body." This phenomenon was called "non- 

 disjunction." Breeding experiments showed that an X-chromosome 

 gene could not be the cause of the phenomenon, and the prediction 

 was accordingly made that half the daughters of a non-disjunctional 

 female would be found to contain in addition to the two X-chromo- 

 somes a supernumerary chromosome which would be a Y. Cytologi- 

 cal investigations have shown that approximately one-half of the 

 daughters of a non-disjunctional female do, in fact, contain a super- 

 numerary F-chromosome, while the remaining half contain only the 

 two X'-chromosomes. I may add that thi'ough the kindness of Dr. 

 Bridges, I have been able to examine some of his slides and convince 

 myself of the presence of the extra chromosome. This brilliant piece 

 of work makes it very hard to disagree with Bridges's conclusion ('14, 

 p. 109) that, "there can be no doubt that the complete parallelism 

 between the unique behavior of the chromosomes and the behavior of 

 the sex-linked genes and sex in this case means that the sex-linked 

 genes are located in and borne by the sex-chromosomes." 



Returning now to a consideration of the linear-arrangement hypo- 

 thesis, it must be admitted that the theory has attractive possibilities, 

 and up to the present time has stood the test of experimental breeding 

 in Drosophila. It may not be out of place, therefore, to call attention 

 in this connection to the constancy of the granular, or chromomeral 

 organization of the chromosomes of Phrynotettix, particularly in 

 chromosome-pair B. May not this constancy of architecture of the 

 chromosomes have a meaning correlated with that assumed in the 

 linear-arrangement hypothesis? This possibility seems to me to be 

 worthy of further investigation. 



