58 Overton —On the Organization of the Nuclei in the 
Zacharias, E. (’95) : Ueber das Verhalten des Zellkerns in wachsenden Zellen. Flora, vol. lxxxi, 
pp. 217-66. 
Zoja, R. (’95) : Sulla independenza della cromatina paterna e nel nucleo delle cellule embrionali. 
Anat. Anz., vol. xi, pp. 289-93. 
Zur Stassen, O. (’98) : Ueber die Riesenbildung bei Ascaris- Eiern. Arch. Entw.-Mech., vol. vii, 
642-76. 
DESCRIPTION OF FIGURES IN PLATES I, II, AND III. 
Illustrating Prof. Overton’s paper on the organization of the nuclei in the pollen mother-cells of 
certain plants. 
All the figures were drawn with the aid of a camera lucida, and with a Bausch and Lomb 1/1 2 
oil immersion or with a Zeiss apochromatic 2 mm. objective. 
PLATE I. 
Thalictrum purpurascens. 
Fig. 1. Cell from anther-wall showing prochromosomes in pairs, linin framework and linin 
intervals. 
Fig. 2. Very young pollen mother-cell showing prochromosomes. 
Fig. 3. More highly magnified nucleus of young pollen mother-cell, in which prochromosomes 
are shown arranged in pairs with parallel linin intervals. 
Fig. 4. Somewhat older pollen mother-cell than is shown in Fig. 2. Prochromosomes in pairs. 
Fig. 5. More highly magnified nucleus of pollen mother-cell in about same stage as shown in 
Fig. 4. Paired prochromosomes, linin intervals and linin framework. 
Fig. 6. Nucleus just previous to synaptic contraction. 
Fig. 7. About same stage as shown in Fig. 6. The prochromosomes are arranged more on 
one side of the nucleus. 
Fig. 8. Synaptic contraction has begun. The chromatin of the prochromosomes is somewhat, 
spread out along the linin intervals. 
Fig. 9. Highly magnified nucleus in early stage of synaptic contraction. 
Fig. 10. Complete synaptic contraction. 
Fig. 11. Complete synaptic contraction viewed more from above than Fig. 10. Prochromosomes 
and linin threads still in pairs. 
Fig. 12. Synaptic mass loosening up. The chromatin of paired prochromosomes is somewhat 
spread out along the linin intervals. 
Fig. 13. Portion of very young post -synaptic spirem, showing the relation of two consecutive 
pairs of prochromosomes to each other. 
Fig. 14. Early stage of uniformly distributed spirem. Bivalent chromosomes with linin 
intervals still distinct. 
Fig. 15. Somewhat older spirem stage than is shown in Fig. 14. 
Fig. 16. Early diakinetic stage showing bivalent chromosomes with parallel linin intervals. 
Fig. 17. Diakinetic stage. Somewhat older nucleus than is shown in Fig. 16. 
Fig. 18. Still older diakinetic stage. Linin connexions still present. 
Fig. 19. Diakinetic chromosomes distinctly shortened, with linin intervals less distinct. 
Fig. 20. Diakinetic nucleus during early stage of spindle formation. 
Fig. 21. Highly magnified figure of two bivalent diakinetic chromosomes still connected by 
linin intervals. 
Fig. 22. Diakinetic chromosomes: 22 a. The univalent portions lie parallel. 22 b. The 
univalent portions are slightly twisted about each other. 22 c. Univalent portions separated in the 
middle, but still attached at the ends. Longitudinal splitting is visible in chromosome at the right 
of this figure. 22 d. The two univalent chromosomes have shortened, forming parallel double rods. 
22 c. Same as Fig. 22 d, but the two rods have somewhat separated at one end. 
