Davis.—Cytological SIndies on Oenothera . II. 645 
tion of threads following synapsis, interpreted by him as a splitting of the 
spireme, which is later closed by the union of the two threads. While it is 
not difficult in these stages and earlier to find threads that run closely 
parallel with one another, the structures are so minute, and the coiled 
arrangements of the threads so intricate, that the writer is not convinced 
that these relations signify more than such an association as would naturally 
result from the contraction of a very complicated thread system. 
The events of the synaptic contraction and of the presynaptic phases 
are probably of great significance, but the research so far upon Oenothera 
does not give a satisfactory explanation. All that can be said is, that from 
a dense reticulum, which develops during the presynaptic stages, there is 
differentiated a complicated thread system that during the synaptic con¬ 
traction lies in intricate coils, and that from this thread system, following 
synapsis, is developed the very much shorter and thicker spireme. It is 
evident that during synapsis the threads become shorter and thicker, but it is 
not clear whether or not the synaptic contraction is merely the result of this 
process or has some deeper significance. Although the threads are reason¬ 
ably distinct during synapsis, their relation to one another is obscured by 
the presence of substances either entangled in the thread system or forming 
with the threads a reticulum. 
It seems to be established for Oenothera that the chromosomes of the 
heterotypic mitosis are not formed side by side through the parallel associa¬ 
tion of two spiremes, but are developed from a single spireme, which by seg¬ 
mentation forms a chain of fourteen chromosomes arranged end to end. 
This fact makes it unnecessary to assume that the synaptic contraction has 
for its purpose the bringing together of two distinct spiremes (of maternal 
and paternal origin), and it is at least possible that the peculiarities of 
synapsis may have no deeper significance than the contraction and con¬ 
densation of the chromatin on a very long and complicated system of 
threads. It is not easy to understand how the chromosomes of the chain 
could be arranged with such precision that they are alternately of maternal 
and paternal origin, which would be the logical arrangement if it be assumed 
that the chromosomes are distributed to give pure germ cells. The ring- 
shaped pairs of chromosomes characteristic of the heterotypic mitosis of 
Oenothera grandijlora evidently result from the close association of adjacent 
segments in the loops of the spireme, and similar pairs when present in 
other species of Oenothera have probably the same method of origin. 
It is clear that the evidence from this study, in agreement with the 
conclusions of Gates and Geerts, supports the theory of the end-to-end 
arrangement, following synapsis, of the full set of sporophytic (somatic) 
chromosomes through the segmentation of a single spireme, as held by 
Farmer and Moore (’ 05 ), Mottier (’ 07 , ’ 09 ), Strasburger (’ 04 ), and their fol¬ 
lowers. The fact that many of the chromosomes in Oenothera biennis , 
Y y 
