Davis . — Cytological Studies on Oenothera . /. 559 
mitosis, when two much thickened halves of a ring (each a sporophytic 
chromosome of this bivalent structure) become clearly defined, connected 
at two points by very delicate strands (PI. XLII, Fig. 37). 
When groups of chromosomes are viewed in favourable positions, two, 
three, or more rings may sometimes be seen actually linked together, thus 
forming short chains or clusters of rings. The appearance of these linked 
chromosomes is shown in Figs. 29 and 30, and there seems to be no doubt 
of the condition, since careful focussing enables one to follow the parts of 
the links above and below one another. 
An explanation of the groups of linked rings seems to require their 
origin from an involved tangle of loops, united at certain points, which 
finally segment in such a manner that the fused loops (now rings) are 
linked together. Such an arrangement may be constructed from a piece of 
string (representing a spirem), loops of which are passed through one 
another and then tied together by thread ; finally, if the string be cut at 
the proper points the loops will be found to separate as linked rings. 
The groups of linked rings remain together until the metaphase of the 
heterotypic mitosis, when they become separated as the halves of the rings 
(sporophytic chromosomes) are drawn apart. Such complicated arrange- 
ments of the rings on the equatorial plate are probably sometimes responsible, 
for a confused cluster of chromosomes that is difficult to interpret, as illus- 
trated by Fig. 35. 
In concluding the account of synapsis and of the formation of the 
bivalent chromosomes, certain striking features should be noted that are of 
importance in relation to the hypotheses that have been offered in discus- 
sions of the structure and origin of heterotypic chromosomes : — 
(1) The spirem at no period lies freely in the nucleus in such a form 
that it may be followed for any considerable portion of its length. It 
is from the beginning an involved coiled structure, the loops of which 
adhere to one another at points especially in contracted portions of the 
synaptic knot. 
(2) The spirem does not segment into fourteen chromosomes, lying 
freely in the nuclear cavity or later becoming associated in pairs. 
(3) The chromosomes appear at the outset as seven pairs (bivalent 
chromosomes) which have the form of generally closed rings, and remain so 
united in pairs until metaphase of the heterotypic mitosis. 
(4) The origin of some of the rings (bivalent chromosomes) from 
loops of the spirem extending freely into the nuclear cavity indicates that 
the halves (sporophytic chromosomes) are segments of the spirem arranged 
end to end, which are brought together in ring-shaped pairs by the coiled 
arrangement of the spirem. 
(5) The occasional presence of linked rings indicates that loops of the 
spirem sometimes become interlaced in such a manner that after the 
