124 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Drosophila, it must be admitted that they go very far towards estab- 
lishing a direct relationship between the chromosomes and the trans- 
mission of Mendelian characters. Perhaps the most convincing evi- 
dence of this kind is that obtained by 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 X Y and that of the female is XX. Furthermore, he found 
that such exceptionally produced females inhent 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 X-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 Y-chromosome, while the remaining half contain only the 
two X-chromosomes. I may add that through 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. 
