546 Gates .— The Trisomic Mutations of Oenothera . 
way. It appears probable that in Oe. lata (Gates and Thomas, 1914 ) the 
three chromosomes belonging to a particular pair (A A x A 2 ) may all under¬ 
go synapsis with each other equally according to chance. Bridges ( 1916 ) 
has shown that in XYY males of Drosophila the synapses of these three 
chromosomes are probably according to chance, while in XX Y females 
homosynapsis occurs much oftener than heterosynapsis. 
In this mutant from Oe. rubricalyx *: Hewettii the unpaired chromosome 
is sometimes clearly indicated (see Fig. 5). Here we must suppose that four 
at least of the ring-pairs were made up of one chromosome from rubricalyx 
paired with the corresponding one from Hewettii. So far as can be judged 
from analogy, another pair might equally be made up from the descendants 
of the original non-disjoined Hezvettii pair or from one of these and the 
corresponding rubricalyx chromosome. The odd chromosome is then likely 
to be a different one in different pollen mother-cell nuclei of this mutant 
according to how synapsis has taken place. But in any case it is clear that 
all these ring-pairs but one must be constituted from a union between 
a rubricalyx chromosome and a Hewettii chromosome, unless we make the 
very unlikely assumption that in this hybrid the pairing of homologous 
chromosomes does not take place. It is therefore impossible to regard the 
mere presence of ring chromosomes as evidence for a homozygous condition 
of the species. It is rather a useful indication of the strength of the attrac¬ 
tion which produces ring-pairs, but it appears that that attraction may on 
occasion be quite as great between the chromosomes of forms belonging to 
distinct species as between the chromosomes of a homozygous species. 
The chromosome rings may persist on the multipolar heterotypic spindle, 
although, as shown in Figs. 3 and 4, the number of rings is usually fewer at 
this time. Occasionally a ring is found persisting even in the heterotypic 
metaphase (Fig. 6), but this is unusual. That one or two ring-pairs of 
chromosomes are occasionally formed in diakinesis and may persist on the 
heterotypic spindle in Oe. Lamarckiana and Oe. lata is shown in an early 
paper (Gates, 1907 , Figs. 17-20, 33, 34), although they were differently inter¬ 
preted at the time. There appears to be no fixity in the number of such 
rings except in forms in which the attraction is so strong that all the pairs 
form rings. 
One striking feature of this 15-chromosome mutant is the fact that the 
pollen is not conspicuously sterile, although the fact that only 33 seeds 
were produced from the self-pollination of a flower shows that a much 
higher percentage than usual of the pollen was non-functional. The irregu¬ 
larities in meiosis are much less numerous than in the various lata forms (cf. 
Gates and Thomas, 1914 ). They apparently begin in the heterotypic 
anaphase through chromosomes lagging behind, as in Fig. 7, where seven 
chromosomes are arriving at one pole, five at the other, and three are 
lagging behind. Fig. 8 is a heterotypic telophase in which apparently eight 
