• Davis.—Cytological Studies oil Oenothera. II. 
649 
EXPLANATION OF FIGURES IN PLATES LII AND LIII. 
Illustrating Prof. Davis’s paper on the reduction divisions of Oenothera biennis . 
All figures were sketched with the aid of a camera lucida under the Zeiss apochromatic objective 
J«5mm. (num. aper. 1*30) in combination with the compensating ocular No. 12, giving a magnifica¬ 
tion of 2,000 diameters. Figure 1 from section 4/1 thick ; Figures 2-11, 24, 25, 27-36, and 42, from 
sections 5 n thick; 12-23, 26, 37 - 4 J > an ^ 43-66, from sections 6 n thick. Sections stained with 
iron-alum haematoxylin. 
PLATE LII. 
Oenothera biennis. 
Figs. 1-32. Illustrating reduction phenomena in the pollen mother-cell. 
Fig. 1. Resting nucleus in pollen mother-cell, showing a large and a small nucleolus, chromatic 
bodies, and delicate reticulum. 
Fig. 2. Similar to Fig. 1, but with a denser reticulum. 
Fig. 3. Nucleus neighbouring to that shown in Fig. 2, the strands of the reticulum becoming 
thicker. 
Fig. 4. Two nuclei separated by a single cell : a , with a delicate reticulum ; b , with a much 
coarser network. 
Fig. 5. Nucleus filled with a dense reticulum shortly before the advent of synapsis, chromatic 
bodies no longer distinguishable. 
Fig. 6. The beginning of the synaptic contraction. 
Fig. 7. Synaptic contraction well under way, the threads and meshes of the reticulum drawn into 
a close mass. 
Fig. 8. A closely contracted synaptic knot. 
Fig. 9. A synaptic knot with loops and threads extending into the nuclear cavity. 
Figs. 10 and 11. Nuclei from opposite ends of the same pollen chamber. Fig. 10, from lower 
end, the threads beginning to shorten and thicken to form the spireme. Fig. 11, from upper end, the 
spireme which emerges from the synaptic contraction following the shortening and thickening of the 
thread system. 
Fig. 12. A spireme with constrictions which show that the process of segmentation has begun. 
Fig. 13. A segmented spireme. 
Fig. 14. The segments of the spireme by condensation of the chromatin have become much 
shorter and thicker. 
Figs. 15 and 16. The segments of the spireme are more nearly the size of the chromosomes 
present at metaphase of the heterotypic mitosis (compare with Figs. 20-23) ; their end-to-end 
arrangement is still clearly evident. 
Figs. 17 and 18. The chromosome segments in about the same stage as shown in Figs. 15 and 
16, but gathered in a close mass which bears a superficial resemblance to a synaptic knot. This is 
apparently the stage termed by some authors a * second contraction 
Fig. 19. Prophase of the heterotypic mitosis. Most of the fourteen chromosomes in the centre 
of the multipolar spindle are united end to end, nucleolus no longer present. 
Fig. 20. Spindle taking on a bipolar form, many of the chromosomes still arranged end to end. 
Fig. 21. Bipolar spindle, the fourteen chromosomes gathered in an irregular group, certain of 
them apparently associated in pairs. 
Fig. 22. Metaphase of the heterotypic mitosis. The chromosomes, now bent in the form of 
thickened V’s, show no very close association in pairs. 
Fig. 23. Anaphase of the heterotypic mitosis. Seven chromosomes are in each of the two sets 
which pass to the poles of the spindle. 
