9 6 3 
. Oenothera Lamarckiana and O. gigas. 
of the resting nucleus when the chromatic material is distributed over 
a network so delicate as so far to defy morphological analysis, but such 
negative evidence in view of the minuteness of the structures in Oenothera 
has, in the writer’s opinion, little or no value. 
The history of the chromosomes in Oenothera during the interkinesis 
between the heterotypic and homotypic mitoses presents very important 
evidence in favour of the theory of the individuality of the chromosomes of 
which the conception of chromosome centres or prochromosomes in the 
resting nucleus forms a part. The writer knows of no plant types in which 
the chromosomes may be more easily traced through this short resting 
period than in Oenothera . The paired chromosomes characteristic of this 
interkinesis may be followed with absolute precision, and this stage affords 
perhaps the most favourable opportunity in the entire life-history of making 
the chromosome count. The process of expansion following the anaphase 
of the heterotypic mitosis increases the size of the chromosomes, but 
alveolization does not set in to such a degree as to obscure their out¬ 
lines, although there are sometimes slight tendencies to form reticula such 
as are conspicuously developed in many plants (e. g. Lilium ) at the same 
stage. It is possible that the period of the interkinesis is too short to 
allow of the complete vacuolization of the chromosomes, or perhaps their 
size and form make their modification more difficult. As a result the 
conditions usual in the mid-period of a resting nucleus have apparently 
been dropped out of this interkinesis in Oenothera ; the process of alveo¬ 
lization never proceeds far, the chief change of chromosome form being 
a slight enlargement followed shortly by the condensation in preparation 
for the homotypic mitosis. 
The most difficult periods of the reduction processes as regards both 
observation and interpretation are those which lead to the differentiation 
of the chromosomes in preparation for the first, or heterotypic, mitosis. 
These periods include the synaptic and presynaptic stages leading to the 
condition called (when present) ‘ diakinesis ’, and the peculiar phenomenon 
of the £ second contraction ’. It has become necessary to define one’s 
usage of the term 4 synapsis ’ since Me Clung (’05) has proposed the restric¬ 
tion of the term to the ‘ fusion of simple chromosomes into multiple ones 
usually of bivalent value ’. Chromosomal associations are the exception 
rather than the rule in the Oenotheras, an important peculiarity which will be 
discussed presently. Me Clung has proposed the term ‘ synizesis ’ for ‘ the 
condition of the nucleus in which the chromatin is found massed at one 
side of the vesicle, without regard to whether it is a normal phenomenon 
or not’. Without questioning the value of McClung’s discrimination of 
the stages in the Orthopteran material studied by him there does not 
seem to the writer sufficient reason for the limitation in plant cytology 
of the term ‘ synapsis ’ to chromosomal fusions, since such phenomena, even 
