346 Mottier . — The Development of the Heterotypic 
Schaffner, J. H. (’97) : The division of the macrospore nucleus. Bot. Gaz., xxiii, 430-52, 1897. 
— — (’06) : Chromosome reduction in the microsporocytes of Lilium tigrinum. 
Bot. Gaz., xli, 184-90, 1906. 
Strasburger, E. (’04) : Ueber Reductionsteilung. Sitzber. Konig. Preuss. Akad. Wiss., xviii, 
587-614, 1904. 
(’05): Typische und allotypische Kerntheilung. Jahrb. f. wiss. Bot., xlii, 1— 71, 
1905. 
Wilson, E. B. (’06) : Studies on chromosomes. III. The sexual differences of the chromosome- 
groups in Hemiptera, with some considerations on the determination and inheritance of sex. 
Journ. Exp. Zool. iii, 1-40, 1906. Important zoological literature is here cited. 
EXPLANATION OP" PLATES XXVII AND XXVIII. 
Illustrating Professor Mottier’s paper on Heterotypic Chromosomes. 
All figures were drawn from sections with the aid of the Abb6 camera lucida and with Zeiss 
apochromatic immersion 2 mm., apert. 1.40, and compensating ocular 12. Magnification x 1800. 
Podophyllum peltatum. 
Fig. 1. Pollen mother-cell prior to synapsis. The chromatin is distributed in the linin net in 
the form of granules of medium size ; the threadwork of the cytoplasm is very distinct. 
Fig. 2. A nucleus of pollen mother-cell from the same anther. The chromatin appears in 
larger masses or lumps, having chiefly a peripheral position ; the nucleolus lies in the centre of the 
cavity which is freer from the linin net. 
Figs. 3 and 4. Cells just prior to synapsis, taken from the same anther loculus. In Fig. 3 the 
chromatin granules are collected into shreds, suggesting a spirem-like condition ; in Fig. 4 the 
chromatin consists of small granules evenly distributed throughout the linin net. The cytoplasm 
is finer mesh than in Fig. 3. 
Fig. 5. Complete synapsis. The entire nuclear contents are contracted into a dense mass. 
The cells show a complete connexion at this stage. 
Fig. 6. The synaptic mass beginning to loosen up. The longitudinal fission is seen in the free 
parts of the spirem. At this stage the cells show the first signs of rounding off at the corners. 
Fig. 7. The loose or hollow spirem. At certain places the halves of the longitudinally split 
spirem diverge slightly. 
Fig. 8. A later stage of the hollow spirem ; the section includes a large part of the nucleus. 
The spirem consists of a rather smooth cord of uniform thickness, in which, as a rule, no trace of 
the longitudinal split can be seen. In this figure the spirem has begun to arrange itself into the long 
loops. Between Figs. 7 and 8 we have a more regular spirem, showing fewer loops. 
Fig. 9. The rearrangement or second contraction. The two limbs of each loop have approxi- 
mated side by side, and some have become greatly twisted about each other. In some cases the 
longitudinal split can be seen in each limb of the loop. Cross segmentation has begun. 
Fig. 10. Second contraction complete, as is also the cross segmentation. The bivalent chromo- 
somes tend to radiate from the centre where their inner ends are much entangled. Either the two 
free ends or the head of the loops may lie toward the nuclear membrane. Preparation for the 
spindle formation is indicated by the presence of kinoplasmic fibres radiating from the nucleus. 
Fig. 11. A later stage in formation of spindle, showing several bivalent chromosomes scattered 
within the spindle complex. The longitudinal split in each segment is no longer apparent. 
Fig. 12. A later stage ; chromosomes being arranged in nuclear plate. 
Fig. 13. Mature spindle, showing forms of chromosomes commonly observed. 
Fig. 14, a, b , c f d. Some of the usual forms of chromosomes met with. 
