Oenothera Lamarckiana and O. gigas. 951 
pollen mother-cell may lie side by side or at right angles to one another. 
The chromosomes, by condensation during the prophases, become very 
much smaller than in the expanded conditions of the interkinesis, and 
by the time that the homotypic spindle is fully developed they have 
returned to about the same size as when they entered the period of inter¬ 
kinesis. They have the form of short and slightly bent rods (Fig. 36). 
During the anaphase of the homotypic mitosis (Fig. 37) and later when 
the chromosomes are gathered at the poles (Figs. 38-40) their form becomes 
more irregular, changing even more markedly after the organization of the 
daughter nuclei which are to enter the pollen-grains. These nuclei (Figs. 41 
and 42) show at first clearly the seven chromosomes which enter into their 
composition, but the form of the chromosomes becomes attenuated as the 
nuclei enlarge. Presently a network becomes evident connecting the 
chromosomes, and the nuclei of the pollen-grains thus pass into typical 
resting conditions (Fig. 43) possessing one or more nucleoli and a delicate 
open reticulum in which lie deeply staining regions. These have the appear¬ 
ance of the chromatin bodies described for the nuclei preceding synapsis, 
but they are of course in the reduced number seven. The chromatic 
bodies become less and less evident as the nuclei increase in size, and by 
the time that the young pollen-grains are formed they can no longer be 
distinguished in the chromatic reticulum that has developed, unless some 
of the denser granules present may represent such chromatic centres or 
prochromosomes.; such granules cannot, however, be counted with regu¬ 
larity. 
This description of the homotypic mitosis in Oenothera Lamarckiana is 
in agreement in all essentials with the account of Geerts for the same 
form, with that of Gates for rubrinervis , and with the writer’s accounts 
of biennis and grandijlora . The mitosis is clearly an equation division 
distributing the halves of chromosomes, the premature division of which 
takes place during the anaphase of the heterotypic mitosis, the halves 
remaining associated in seven pairs throughout the period of interkinesis. 
The Partial Sterility of the Pollen in Oenothera Lamarckiana. 
De Vries noted the fact of partial sterility in Lamarckiana of both pollen- 
grains and ovules, and this subject is given considerable attention in the 
account of Geerts (’ 09 , pp. 82-105), who reports that about 50 per cent, 
of these structures may be abortive. Geerts figures a number of pollen- 
tetrads and young embryo-sacs in which the nuclei give evidence of 
degeneration, but he does not clearly associate the phenomenon with 
irregularities in the distribution of the chromosomes. 
Gates (’ 07 ) presents observations on the development of the pollen 
in the hybrid lata x Lamarckiana, showing clearly that the group of seven 
chromosomes which normally should enter each nucleus of the pollen-grain 
may be broken up with the result that extra nuclei are formed having 
