970 Davis.—A Comparison of the Reduction Divisions of 
nature. The chromosomes take on their form and proportions by a shorten¬ 
ing of the spireme segments marking the end of the long process of chromatic 
condensation which began in presynaptic stages. 
7. The condensation of the spireme segments results in a further 
shortening of the spireme during which much of its complicated looped 
arrangement disappears. Associated with this condensation is a second 
contraction during which the segments of the spireme or chains of chromo¬ 
somes are frequently drawn closely together in a cluster. It is doubtful 
whether this stage is of especial significance since the chromosomes are not 
regularly grouped in pairs, and it may be merely the result of the final 
condensation of the chromatin previous to the differentiation of the fully 
developed chromosomes. 
8. The chromosomes separate during the prophases of the heterotypic 
mitosis, but, although adjacent chromosomes sometimes form rings, there is 
no general pairing of the chromosomes characteristic in some forms of the 
stage of diakinesis. The chromosomes by contracting and bending in the 
middle regions become V-shaped ; they are all essentially similar to one 
another. 
9. When grouped in pairs it is clear that such alternate (adjacent) 
chromosomes on the spireme are separated by the heterotypic mitosis, but 
the generally scattered arrangement makes it impossible to determine 
whether alternate chromosomes are always so distributed. The absence of 
a conspicuous stage of diakinesis with the chromosomes grouped in pairs 
makes the determination of this important point very difficult in biennis , 
Lamar ckiana, and gigas. 
10. The heterotypic mitosis is a reduction division distributing the 
fourteen chromosomes of Lamarckiana and the twenty-eight chromosomes 
of gigas in two sets of seven and fourteen chromosomes each for the 
respective species. 
11. A lengthwise fission of each chromosome in preparation for the 
homotypic mitosis becomes apparent during the anaphase of the hetero¬ 
typic division. 
12. Thus seven split chromosomes in Lamarckiaria and fourteen split 
chromosomes in gigas enter the nuclei of the interkinesis between the 
heterotypic and homotypic mitoses and may be followed as pairs of 
chromosomes throughout the period of the interkinesis. 
13. The homotypic mitosis is an equation division distributing the 
members of the pairs of chromosomes present during the interkinesis. 
14. Irregularities in the distribution of the chromosomes are not 
uncommon, and may account for a portion of the sterility of the pollen in 
these forms, but it seems probable that other factors of a physiological 
nature are operative. 
15. The pollen mother-cells and their nuclei and also the pollen- 
