Davis.—Cytological Studies on Oenothera . //. 639 
a nucleus are as clearly defined as in Figs. 29 and 30. The daughter nuclei 
remain for some time in this resting condition with the seven pairs of chromo¬ 
somes distributed around the periphery and with one or more nucleoli 
present. It is clear that the chromosomes of biennis maintain their 
individuality throughout the interkinesis, and their behaviour in this 
respect is in complete accord with the history during the same phase 
in grandiflora (Davis), Lamarckiana (Geerts), and rubrinervis (Gates). 
The chief conclusions from this study of the heterotypic mitosis 
of Oenothera biennis are (1) that fourteen V-shaped sporophytic (somatic) 
chromosomes are distributed in two groups of seven each, so that the mitosis 
is a reduction division ; (2) that the chromosomes are assembled irregularly 
at the equatorial plate, and there is no uniform groupingof the structures in 
pairs, although occasional pairs may be found ; (3) that the chromosomes are 
essentially similar to one another in form and size; and (4) that during 
anaphase there is a fission of the chromosomes, so that seven split chromo¬ 
somes enter each daughter nucleus and are evident as seven pairs during the 
period of interkinesis. 
The Homotypic Mitosis. The approach of the homotypic mitosis 
is indicated, as in the heterotypic division, by the appearance of a delicate 
web of fibrillae around the resting nucleus of the interkinesis (Fig. 30). 
With the breaking down of the membrane the fibrillae enter the nuclear 
cavity (PI. LIII, Fig. 54, prophase of homotypic mitosis in the ovule) and 
establish a multipolar spindle which quickly changes into a bipolar structure. 
The seven pairs of chromosomes are carried towards the centre of the 
developing spindle (Figs. 31 and 32), where they form an irregular cluster. 
During these changes the chromosomes by condensation become much 
smaller than in the expanded condition of the interkinesis, and by the time 
the homotypic spindle is fully developed (PI. LIII,Fig. 33) they have returned 
to about the same size as when they entered the period of interkinesis. 
The pairs of chromosomes are arranged quite regularly on the equatorial 
plate of the homotypic spindle, so that as a result the members of each pair 
are separated by the mitosis. The two spindles may lie side by side or at 
right angles to one another (Fig. 33). 
The form of the chromosomes at metaphase of the homotypic mitosis 
is that of short and sometimes slightly bent rods, but during anaphase 
(Fig. 34) one or both ends generally enlarge somewhat, so that the outline 
becomes irregular. The irregularities of form grow more pronounced after 
the organization of the daughter nuclei (Fig. 35), in which a chromatic 
network is finally formed by the elongation, branching, and anastomosis of 
the chromosomes. The nucleus of the young pollen grain thus passes into 
a typical resting condition (Fig. 36) possessing one or more nucleoli and 
a delicate open reticulum in which lie thickened deeply staining regions. 
The latter are frequently so definite in outline that it seems probable they 
