Pollen Mother-cells of Lilium canadense. 
257 
DESCRIPTION OF FIGURES IN PLATES VI, VII, VIII, AND IX. 
Illustrating Prof. Allen’s paper on the Pollen Mother-cells of Lilium canadense. 
All the figures were drawn with the aid of a camera lucida, and with a Zeiss apochromatic 
2 mm. objective, 1.30 apert. : Figs. 15, 16, 17, 18, 24, 27, and 31 with compens. oc. 18; all the 
others with compens. oc. 12. 
PLATE VI. 
Fig. 1. Reconstructing nucleus of pollen mother-cell, shortly after close of last pre-heterotypic 
division. Spindle fibres (not shown) still attached on equatorial side (lower side in figure). 
Fig. 2. Somewhat older nucleus, showing increased size ; irregular nucleolar masses. 
Fig. 3. Tangential view of portion of nuclear reticulum, same stage as Fig. 2. 
Fig. 4. Pollen mother-cell at slightly later stage. 
Fig. 5. Still older nucleus ; peripheral arrangement of chromatic material; rounded, vacuolated 
nucleole. 
Fig. 6, Tangential view of portion of nuclear reticulum, same stage as Fig. 5. 
Fig. 7. Median section of still older nucleus, shortly before synapsis ; beginning of formation of 
spirem. 
Fig. 8. Tangential view of nucleus, same stage as Fig. 7. 
Fig. 9. Portion of nuclear reticulum showing paired fibres ; same stage as Figs. 7 and 8. 
Figs. 10, 11. Stages in the transition to synapsis. 
Fig. 12. Nucleus in synapsis, tangential view; spirem composed of two parallel threads; no 
free ends ; flattened nucleole. 
Fig. 13. Portion of double spirem, same stage as Fig. 12. 
Fig. 14. Median section of cell with nucleus in synapsis ; paired strands at a. 
Figs. 15-18. Stages in the fusion of the parallel threads; substance of threads differentiated into 
chromatin and linin. 
Fig. 19. Late synapsis stage, with single thick spirem ; cell has separated from its fellows and 
rounded up. 
Fig. 20. Cell showing stage in transition from synapsis. 
PLATE VII. 
Fig. 21. Later stage in transition from synapsis. 
Fig. 22. Median section of cell with uniformly distributed spirem. 
Fig. 23. Nearly complete nucleus, showing course of spirem, at same stage as Fig. 22. The 
free ends visible are all in the plane of cutting. 
Fig. 24. Portion of thread, stage of Fig. 23, showing single row of chromomeres. 
Fig. 25. Portion of tangential view of nucleus, showing longitudinal splitting of the spirem. 
Fig. 26. Median section of cell, same stage as Fig. 25. 
Fig. 27. Portions of spirem, showing process of longitudinal splitting. 
Fig. 28. Early stage in segmentation ; a few free ends, all in the periphery. 
Fig. 29. Later stage in segmentation. 
Fig. 30. Nucleus some time after segmentation ; chromosomes mostly in the periphery and 
somewhat shorter and thicker than in Fig. 29. 
Fig. 31. Two chromosomes, same stage as Fig. 30 ; two rows of chromomeres in each daughter 
chromosome; traces of second longitudinal splitting at a and b. 
Fig. 32. Nucleus just before formation of multipolar spindle; short, thick chromosomes. 
Fig- 33 * Chromosome at time of completion of segmentation. 
Fig s * 34? 35- Chromosomes, showing early stages in the process of shortening and thickening. 
