Voi,. 7, 1921 
GENETICS: LANCEFIELD AND METZ 
227 
The lack of inequality in the sex chromosomes of the willistoni male 
necessitated obtaining non-disjunctional flies with three sex chromosomes 
in order to distinguish sex chromosomes from autosomes. As shown by 
the genetic data given below these non-disjunctional flies were females 
having one Y and two X chromosomes. Four chromosome groups from 
such females are shown in figures 7-10. In each of these there is an extra 
chromosome, and it is clearly a large and V-shaped one. Since this must 
be a sex chromosome it proves that the sex chromosome pair in willistoni 
is one of the long, V-shaped pairs and not the rod-like pair as would have 
been expected from analogy with melanogaster. We have obtained 
numerous clear cut figures both of the normal group in each sex and of 
the non-disjunctional group, and are confident that the evidence is en- 
tirely conclusive on this point. 
Genetic Data. — In obtaining and identifying the non-disjunctional 
flies we have followed the procedure used by Bridges ('16) in the case of 
D. melanogaster. The following summarized account shows the resem- 
blance between the results in the two species and indicates the source of 
the XX Y chromosome groups described above. 
Primary non-disjunction appears to be very rare in willistoni and no 
cases (which could be checked) were observed in our regular experiments. 
In approximately 150 cultures made especially for this purpose only two 
cases were detected. In each of these the exceptional fly was a female. 
The first came from a cross of a rough eyed female by an orange, small- 
bristle male, both from stock cultures. (Rough, orange, and small-bristle 
are all inherited as sex-linked recessives.) This mating gave 132 normal 
daughters, 111 rough eyed sons, and 1 rough eyed (exceptional) daughter 
(Culture W 1715). Since this rough daughter had already mated with a 
rough brother, it was not possible to detect "secondary" exceptions pro- 
duced by her. Six of her rough daughters, however, produced 11 ex- 
ceptional sons and daughters among 960 significant flies, or 1.14 percent 
of exceptions. In succeeding generations about the same ration was main- 
tained, as is shown in table 1. For the sake of convenience, this strain 
has been designated as "line A." 
The second primary exception appeared in an entirely unrelated stock, 
involving the characters two-bristle, short-3, and rough. It gave rise to 
a second strain known as "line B." Table 1 gives a summary of the 
breeding tests with this line, which gave results essentially similar to those 
of line A. 
In addition to the ordinary cases of "reductional" non-disjunction 
we have also detected two cases of "equational" non-disjunction. This 
type was first described by Bridges ('16) who reported 18 "equational ex- 
ceptions," i.e., daughters homozygous with respect to a recessive character 
for which the mother was only heterozygous. In most of his cases, he 
