93 8 Gates.—Pollen Formation in Oenothera gigas. 
Fig. 9. Three pollen mother-cells, showing how the extrusion takes place, x 1,275. 
Fig. 10. The same more highly magnified, x 1,875. 
Fig. 11. Section of pollen mother-cell, showing the two pseudo-nuclei formed by extrusion from 
the adjacent cell. In the one on the right the chromatic material is just beginning to break up, and 
on the left this process has gone further, producing an appearance resembling a spireme. In the next 
section the nucleus of this cell and its cytoplasmic connexions are seen, x 1,875. 
Fig. 12. Chromatin has been extruded from the nuclei to form dark masses in the cytoplasm of 
the adjoining cell. The nuclei are moving back towards the centre of the cell, x 1,875. 
Fig. 13. Showing numerous cytoplasmic connexions between pollen mother-cells, x 1,875. 
Fig. 14. A later stage in which the nucleus occupies a central position. The nuclear thread- 
work has been completely rearranged since synapsis, the threads now occupying the whole cavity, 
and being characteristically straight instead of meandering, x 2,850. 
Fig. 15. A synaptic stage in which most of the threads line the nuclear membrane and are seen 
here in cross-section. A few threads, varying greatly in thickness, pass through the cavity of the 
nucleus, x 2,850. 
Fig. 16. Same stage as Fig. 15. x 2,850. 
Fig. 17. Same stage, differently represented, x 2,850. 
Fig. 18. Section of nucleus at this time, showing only the ‘ chromomeric ’ threadwork lining 
the nuclear membrane. Some free ends of threads appear, x 2,850. 
Fig. 19. Same as Fig. 18, showing also the nucleolus and several other bodies attached to the 
nuclear membrane, x 2,850. 
PLATE LXVIII. 
Fig. 20. Pollen mother-cell in heterotypic metaphase. Twenty-six chromosomes are visible, 
rather irregularly scattered on the spindle, several apparent pairs. x 2,100. 
Fig. 21. Heterotypic spindle in metaphase, showing twenty chromosomes, x 2,100. 
Fig. 22. Heterotypic spindle in early anaphase, showing nineteen chromosomes and a per¬ 
sistent nucleolus, x 2,100. 
Fig. 23. Heterotypic spindle, early anaphase in polar view. Twenty-seven chromosomes 
visible, x 2,100. 
Fig. 24. Heterotypic spindle, late anaphase, slightly oblique cut. , All fourteen chromosomes 
present at lower pole, and twelve at upper pole of spindle, x 2,100. 
Fig. 25. Oblique cut, later anaphase, 14+ 11 chromosomes visible, x 2,100. 
In Figs. 22, 24, and 25 the anaphase chromosomes vary in shape, being mostly sub-globular, 
rod-shaped, or V-shaped, but very few show indications of their bivalent character. 
Fig. 26. Late heterotypic anaphase. Six of the chromosomes show a clear median transverse 
constriction or segmentation, while several others have the characteristic appearance of ‘tetrads’. 
It appears that both transverse and longitudinal divisions of the chromosomes may occur at this 
time, x 2,100. 
Fig. 27. A late stage of interkinesis. The remnants of the heterotypic spindle still remain in 
the cytoplasm. The chromosomes do not always anastomose to this extent during interkinesis, 
x 2,100. 
Fig. 28. Homotypic prophase, showing beginning of spindle formation and chromosome 
bivalents. The mother-cell wall has characteristic outgrowths. See text, p. 924. x 2,100. 
Fig. 29. Slightly later stage than Fig. 28, showing fourteen bivalent chromosomes on the 
multipolar spindle, x 2,100. 
Fig. 30. Slightly later stage of homotypic spindle. The spindle fibres are being rearranged 
and attached to the fourteen chromosomes, only one of which in this case shows its bivalent 
character, x 2,100. 
Fig. 31. Homotypic prophase. The chromosomes are all present, sixteen on one spindle and 
twelve on the other. They vary much in size and shape, but most show their bivalent character. 
This unequal distribution must have occurred in the heterotypic mitosis, x 2,100. 
Fig. 32. Homotypic metaphase. In all the mother-cells of this anther the spindles were very 
narrow, so that there was scarcely room for the chromosomes, x 2,100. 
Fig. 33. Homotypic metaphase in polar view, showing the two scattered groups of fourteen 
chromosomes, x 2,100. 
