642 Davis.—Cytological Studies on Oenothera. II. 
chromosomes, seven in each group, separate to pass to the respective poles 
of the spindle (Fig. 51). The chromosomes are not closely or uniformly 
associated in pairs either during their organization or in their arrangement 
at the equatorial plate. 
The chromosomes divide lengthwise as they pass from the equatorial 
plate to the poles of the heterotypic spindle, so that each daughter nucleus 
following the heterotypic mitosis receives seven split chromosomes (Fig. 52). 
The seven pairs of chromosomes maintain their individuality throughout 
the period of interkinesis (Fig. 53), during which they increase in size. 
Figs. 49-53, outlining the history of the heterotypic mitosis and the 
period of interkinesis in the ovule, may be compared with Figs. 19-30, 
which illustrate in greater detail the same events in the pollen mother-cell. 
The Homotypic Mitosis. The seven pairs of chromosomes present 
during the interkinesis become by condensation much smaller during the 
prophase of the homotypic mitosis (Fig. 54), and have the form of short 
rods by the time that they are arranged on the equatorial plate (Fig. 55, 
compare with Fig. 33). The members of each pair are distributed by this 
mitosis into two sets, seven chromosomes in each, the chromosomes assum¬ 
ing a more irregular outline during anaphase (Fig. 56, compare with 
Fig. 34). With the organization of the daughter nuclei and the develop¬ 
ment of cell plates across the two homotypic spindles, a row of four cells is 
organized homologous with the pollen tetrad. The nuclei of these cells pass 
into a resting condition (Fig. 57, compare with Fig. 36), the chromatin 
forming a reticulum upon which lie chromatic bodies, variable in number, 
some of which are probably prochromosomes. 
The chief stages in the development of the embryo sac were traced, and 
the main features of Geerts’ account were confirmed. There are only two 
mitoses, and these result in a group of two synergids and an egg nucleus at 
the micropylar end of the sac with a single polar nucleus below, antipodals 
therefore being absent. It is very difficult to obtain good fixation of stages in 
the development of the embryo sac and during fertilization and in the early 
segmentation of the fertilized egg, and the writer is not yet ready to take 
up the cytological features of this phase in the life-history of Oenothera. 
As is well known the anthers of Oenothera biennis discharge their pollen 
in the bud, so that the base of the stigma is well covered when the flower 
opens. Sections of the present material showed that the pollen tubes reach 
the embryo sacs even before the opening of the buds. With such con¬ 
ditions it can readily be understood that ordinarily the production of seeds 
by Oenothera biennis through cross pollination is unlikely. Cross pollination 
of this form in nature would appear to be probable only when its own pollen 
is insufficient to provide for the number of ovules matured, a possibility that 
seems remote. 
