646 Davis.—Cytological Studies on Oenothera. II. 
rubrinervis , and Lamarckiana are not closely associated in pairs does not 
affect the main point, which is that they are arranged end to end in a single 
spireme during the prophases of the heterotypic mitosis. The very clear 
association of the chromosomes in pairs shown in the material of O. grandi- 
flora studied by the writer is explained by the more complicated looped 
structure of the spireme in this form, which brings adjacent chromosome 
segments into such intimate relations that they remain together as ring- 
shaped pairs (bivalent chromosomes). The writer can see no possibility of 
explaining the reduction phenomena of Oenothera by the theory of a side- 
by-side pairing of chromosomes through the parallel association of two 
spiremes as held by Gregoire (’ 04 , W), Allen ( 05 ), Rosenberg (’ 05 , ’09 a ), 
Overton (’ 05 , ’ 09 ), and others. 
With respect to the attractive theory of Rosenberg (’ 04 , * 05 ) and 
Overton (’ 05 , ’ 09 ) that chromosomes are represented in the resting nuclei 
by chromatic centres or prochromosomes, this study offers only indirect 
evidence. Chromatic bodies may be readily distinguished in resting nuclei, 
and it seems probable that some of these are prochromosomes, but the 
counts are too variable to establish their identity with the chromosomes. 
Furthermore, the chromatic bodies could not be traced to the chromosomes 
differentiated in the prophases of mitosis since their outlines became lost in 
the chromatic network which precedes the differentiation of the spireme. 
The fact that the chromosomes may be followed through the interkinesis 
between the heterotypic and homotypic mitoses offers strong support to 
the theory of the individuality of the chromosomes, but it must be borne in 
mind that these two mitoses (heterotypic and homotypic) are closely bound 
together as parts of a common process (that of chromosome reduction), the 
phylogenetic relations of which to the events of vegetative mitosis are not 
understood. The writer hoped that material fixed in the fluids of Carnoy, 
as employed by Overton, might give better results for the study of the 
chromatic bodies in resting nuclei and in the early stages of mitosis and 
synapsis, but such material proved to be no more favourable than that killed 
in Flemming’s fluids. 
Summary. 
1. The resting nuclei of the pollen mother-cell and megaspore mother¬ 
cell contain chromatic bodies (variable as to number), some of which are 
probably chromosome centres, or prochromosomes. These structures lie 
in a very delicate reticulum accompanied by one or more nucleoli. 
1 . Shortly before synapsis the nucleus becomes filled with a close 
reticulum, and the chromatic bodies become lost in the deeply staining 
strands of this dense network. 
3. The advent of synapsis is marked by a slow contraction of the 
reticulum away from the nuclear membrane, carrying most of the strands 
