i 9 o8] GATES— REDUCTION IN OENOTHERA $$ 



Figs, i, 2. — Young meristematic cells of anther primordium showing one 

 large nucleolus and several smaller ones, and chromatic masses adherent to the 

 nuclear membrane. 



Fig. 3. — Longitudinal section of anther, showing size relations of nucleoli in 

 sporogenous, tapetal, and wall cells. 



Fig. 4. — One sporogenous cell from stage of fig. 3, previous to synapsis; 

 cytoplasm somewhat vacuolate. 



Figs. 5, 7-9. — Nuclei at same stage, showing fusions of nucleoli. 



Fig. 6. — Two nucleoli of equal size; an unusual condition. 



Fig. 10. — Several small nucleoli, and no indication of fusion. 



Fig. 11. — Nucleoli of young pollen grain nucleus. 



Fig. 12. — Beginning of synaptic contraction; the reticulum has contracted 

 from the nuclear membrane on all sides, leaving several loops attached to the 

 membrane; on the side on which the reticulum retains the curved outline of the 

 nuclear membrane the latter has been drawn inward attached to the threads; on 

 the rest of the circumference, between the loops, the nuclear membrane remains 

 in situ; the cytoplasm is perfectly fixed. 



Fig. 13. — Another contraction stage, showing loops attached to the nuclear 

 membrane, which is intact. 



Fig. 14. — A slightly later stage of contraction, in which the rearrangement 

 of threads is taking place. 



Fig. 15. — Synapsis; dark-staining bodies are still held in the meshes of the 

 spirem; a small nucleolus, usually about the size of a chromosome, is generally 

 present in addition to the large nucleolus. 



Fig. 16. — After synapsis; the thread thicker and shorter and loosely coiled. 



Fig. 17. — Slightly later stage than fig. 16, and less deeply stained; thread 

 shows the characteristic light and dark areas; indications of parallel threads in 

 two places; edge of thread may be even or moniliform. — 5 m. 



Fig. 18. — Later stage; thread much shortened and greatly thickened and 

 entering upon second contraction phase; nucleus uncut. — 10 /". 



Fig. 19. — Higher magnification of a portion of the thread in figs. 20 and 21. 



PLA TE II 



Fig. 20. — Second contraction stage; a pair of chromosomes cut off from 

 spirem; nucleus uncut. — 10 /*. 



Fig. 21. — Second contraction stage; nucleus uncut. 



Fig. 22. — Uncoiling from second contraction stage; pair of chromosomes 

 detached; nucleus uncut. 



Fig. 23. — Spirem segmented in three places, each segment showing constric- 

 tions which will form the chromosomes; certain chromosomes already detached; 

 nucleus uncut. 



Fig. 24. — Constriction of spirem has proceeded farther, the chromosomes being 

 elongated bodies with irregular margins like the spirem, and connected by rather 



