Davis. — -Cytological Studies on Oenothera . /. 
563 
The Homotypic Mitosis. 
The spindle of the homotypic mitosis is formed as in the heterotypic 
division from a mesh of fibrillae that develop around the resting nucleus 
and enter the nuclear cavity with the dissolution of its bounding membrane 
(Fig. 46). The prophases likewise frequently show multipolar spindles 
(Figs. 47 and 48) that become bipolar at the metaphase of mitosis. The 
two spindles may lie parallel in the pollen mother-cell or at right angles to 
one another. 
The chromosome group emerges from the resting nucleus at the 
beginning of the homotypic mitosis in very much the same form as it 
entered that structure at the end of the heterotypic. There are the same 
seven split chromosomes, the halves of which are bent outward so that 
they are joined to one another only in the bent middle region. This 
peculiar chromosome group presents an interesting contrast to the seven 
rings characteristic of the heterotypic mitosis (compare Figs. 46-48 with 
Figs. 34 and 36). 
The seven split chromosomes are gathered in the usual manner at 
the equatorial plate, where they are arranged so that the halves separate 
into two sets at the metaphase of mitosis (Figs. 49 and 50). A comparison 
of this stage with the metaphase of the heterotypic mitosis (Fig. 37) shows 
that the daughter chromosomes are not so large as those of the latter 
mitosis and have the form of short and sometimes slightly bent rods 
rather than of V’s. One or both ends of the chromosomes may become 
somewhat enlarged, giving them an irregular appearance that is frequently 
quite pronounced during anaphase of this mitosis (Fig. 51). 
Each daughter-nucleus following the homotypic mitosis thus receives 
one set of the halves of the seven split chromosomes. These halves may 
be readily counted in favourable preparations of anaphase (Fig. 52), and 
especially when the chromosome group at the pole of the spindle is viewed 
from above (Fig. 53). The arrangement of the chromosomes in late 
anaphase and early telophase is similar to that in the heterotypic mitosis 
(i. e. six chromosomes are grouped in a circle around the seventh), but 
in this group there is not present of course any trace of a premature 
fission in preparation for a succeeding mitosis. 
As the four daughter-nuclei following the homotypic mitosis increase 
in size, the chromosomes which enter into their construction begin to 
elongate and finally to branch. Although they may be distinguished in 
the younger nuclei, such as are shown in Fig. 54, their forms become 
irregular in the older (Fig. 55), where a network is finally developed. The 
nucleus of the young pollen-grain (Fig. 56) generally has several medium- 
sized nucleoli, and the chromatin is distributed in the form of small deeply 
staining bodies connected with one another by a delicate open reticulum. 
Qq 
