641 
Davis.—Cytological Studies on Oenothera . II. 
We shall now outline briefly, and for the sake of comparison with the 
more detailed account of the pollen mother-cell, the history of synapsis and 
of the reduction divisions in the ovule. 
Presynapsis. The resting nuclei of the megaspore mother-cells 
(Fig. 37) show the presence of chromatic bodies similar to those described 
for the nucleus of the pollen mother-cell (Figs. 1-4), but here also it is 
frequently possible to make counts above and below the number fourteen, 
irregularities for which various explanations might be suggested. The 
writer is, however, inclined to believe that certain of these structures are 
prochromosomes. The chromatic bodies are distributed upon a very 
delicate linin reticulum, and wherever two lie close together (Fig. 37) their 
arrangement appears to be end to end upon a strand of the network. 
On the approach of synapsis the delicate reticulum is gradually replaced 
by a dense network of deeply staining threads (Figs. 38 and 39) which fill 
the nucleus (compare with Figs. 4 and 5), and at this stage the chromatic 
bodies cannot be distinguished with certainty. 
Synapsis. A contraction of the dense reticulum (Fig. 40) marks the 
advent of synapsis when the meshes of the network become drawn together 
into a close mass, the synaptic knot (Fig. 41), generally near to the nucleolus. 
After remaining for a considerable time in this contracted condition the 
synaptic knot appears to loosen, when it is evident that the threads of the 
system have become much thicker (Figs. 42 and 43) and begin to resemble 
a much looped spireme. This history of synapsis is illustrated for the 
nucleus of the pollen mother-cell by Figs. 6-10. 
The Formation of the Chromosomes. Following the loosening of the 
coils of the synaptic knot there is a rapid development of an evident spireme 
by the contraction of the thickened threads. The spireme (Fig. 44) is very 
much shorter and thicker than the system of threads that emerges from 
synapsis, and is not so complexly looped. A segmentation of the spireme 
then transforms this structure into a chain of fourteen chromosomes 
(Figs. 45-47, compare with Figs. 12-17). Occasionally the segmented 
spireme or the group of chromosome segments derived from the spireme 
is found so contracted (Fig. 48, compare with Fig. 18) as to suggest con¬ 
ditions of synapsis, but this stage, which some authors would term a ‘ second 
contraction does not appear to be uniformly present, and is not, in the 
opinion of the writer, of especial significance. 
The Heterotypic Mitosis. The fourteen chromosomes, which result 
from the segmentation of the spireme, appear at the equatorial plate of the 
heterotypic spindle in the form of rod-shaped structures (Fig. 49), some 
of which even at this late stage of mitosis are frequently found still arranged 
end to end in short chains. By continued condensation the chromosomes 
are further shortened, and later take on the form of V’s, a condition which 
is most evident at metaphase (Fig. 50), and also when the two sets of 
