566 Davis . — Cytological Studies on Oenothera. J. 
greater difficulties than the post-synaptic events considered above. Not 
only must one deal with more intricate structure but also with conditions 
where the necessities of the best possible technique are far more exacting 
than in the later stages of the reduction processes. The last contribution 
of Overton (’09) offers such important evidence on the structure and 
arrangement of the chromosomes during presynaptic phases in the history 
of the pollen mother-cells that the writer prefers to defer an expression of 
opinion concerning the conditions in Oenothera at these periods until further 
studies have been concluded. 
Overton (’09), from studies on Thalictrum , Calycanthus , and Richardia , 
believes that the chromosomes are represented in the vegetative (somatic) 
nuclei by prochromosomes arranged in parallel pairs separated by linin 
intervals indicating the presence of parental sets of chromosomes associated 
together to form heterogeneous spirems. The synaptic contraction is 
a gradual process during which parental spirems become more closely 
associated but still remain distinct, and the chromosomes of each pair are 
in their most intimate relation during post-synaptic stages. Each pair 
becomes a bivalent chromosome composed of two sporophytic chromosomes, 
the bivalent structures being characteristic of diakinesis. No folding 
process to form these bivalent chromosomes seems possible. The sporo- 
phytic chromosomes of the bivalent diakinetic pairs undergo lengthwise 
divisions, thus forming apparent tetrads. The heterotypic mitosis dis- 
tributes entire sporophytic chromosomes, which remain distinct during the 
following interkinesis. The homotypic mitosis separates the split halves of 
the sporophytic chromosomes, and these may be followed in the pollen-grain 
as prochromosomes arranged in a single row. 
It will be noted at once that Overton’s conclusions offer strong support 
to the theory of chromosome reduction held by Gregoire (’04, ’07), Allen 
(’05), Rosenberg (’05), and certain later writers. This group believe that two 
sets of sporophytic chromosomes of maternal and paternal origin are 
represented in the nucleus by two distinct spirems or sets of threads which 
become closely associated in synapsis, so that by a parallel arrange- 
ment of the spirems the chromosomes of one set are paired with those 
of the other to form the bivalent chromosomes of the heterotypic mitosis. 
Another group of investigators, represented by Farmer and Moore (’05), 
Mottier (’07), Strasburger (’04), and others, hold that the sporophytic 
chromosomes are arranged end to end on a single chromatic thread and 
that the bivalent chromosomes of the heterotypic mitosis are formed from 
loops of this single spirem that include a pair of sporophytic chromosomes 
which thus come to lie side by side. 
The writer, on theoretical grounds, has been very strongly inclined 
towards the views of the first-named group represented by Allen, Gregoire, 
Overton, and Rosenberg. It must, however, be pointed out that the present 
