120 Mottier — Mitosis in the Pollen Mother -cells of 
free extremities. I hold the first alternative as the most probable. In any. 
event, the large ring in Fig. 17 is not formed by the divergence of the 
halves of a longitudinally split spireme with the free ends adhering, for 
there is no longitudinal fission evident, and if the fission really exists this 
fact is not made apparent by the separation of the halves. Furthermore, all 
other evidence is against the view of the separation of longitudinal halves to 
make a bivalent. When the sides of a U are twisted about each other, we 
have the familiar figure 8 form or the hour-glass shape. At one end of the 
8 the free ends frequently overlap, while the other end may represent the 
bend in the loop. A glance at Figs. 15 to 18 shows that all the bivalent 
chromosomes are not rings, U’s, or 8’s. Some are composed of two straight 
or slightly curved rods that may be parallel side by side with or without 
any twisting about each other ; or the two pieces may adhere at one end to 
form V’s, cross each other to form X’s, or two of the free extremities may 
diverge to form Y’s. All of these familiar forms may be found in the same 
nucleus. It sometimes happens also that the two straight somatic chromo- 
somes may lie in a straight line adhering end to end, a phenomenon found 
in various other plants. Now the bivalents that appear as two straight rods 
lying in contact side by side, or that form X’s, &c., did not necessarily arise 
from the spireme as loops, but were in all probability derived from those 
parts of the spireme that were straight, or even from the more closely 
knotted or entangled parts of the chromatin thread. However, they repre- 
sent different lengths of the spireme that have approximated side by side, 
and not the halves of the same piece of the thread that had split lengthwise. 
It follows, therefore, that, if the spireme were split lengthwise, as is the case 
in many plants, each member of the bivalent or somatic chromosome would 
be composed really, if not visibly, of two halves that tend to separate during 
the anaphase, and that do separate from each other during the telophase. 
This separation, which is finally and fully brought about during the second 
division, is the equational division, because the chromosomes separating in 
the second division represent halves of somatic chromosomes produced by 
the longitudinal split. The approximation or the so-called pairing of two 
somatic chromosomes to form bivalents is not understood by the writer as 
a conjugation. The assumption that there is an exchange of material 
taking place at this step in the mitotic process is based upon purely 
hypothetical data. If there is an exchange of material between chromo- 
somes, is there not greater opportunity for such an exchange in the spireme 
itself, or in synapsis, or even in the resting stage, when the chromatin is in 
its most finely divided state ? A further statement of this point will appear 
below. 
In the entire history of the nucleus from the stage of rest to the 
formation of the twelve bivalents, nothing is clearer to the writer than 
the fact that all of the bivalents are derived from the spireme ; that no 
