Pollen Mother -cells of Certain Plants . 33 
to be identified with the longitudinal approximation line of the thick spirem. 
It is an entirely new fission, which splits the univalent chromosomes in half, 
forming four-parted chromosomes. In plants in which the approximation 
line can be identified during the various stages in the development of the 
bivalent heterotypic chromosomes, there is absolutely no danger of confusing 
this split with the approximation line. 
A multipolar spindle is formed which later becomes bipolar, and the 
four-parted chromosomes become oriented in the equatorial plate as in 
Thalictrum purpurascens. Fig. 12, PI. II, is a polar view of an equatorial 
plate stage, in which nearly all of the chromosomes may be seen to be 
four-parted as viewed from the end. Fig. 13, PL II, shows a spindle with 
the chromosomes in the equatorial plate. In each of these figures one 
chromosome is considerably larger than the others. In the last-mentioned 
figure the chromosomes are shown also as four-parted structures, forming 
the characteristic double V’s as I described them for Thalictrum purpur- 
ascens. Figs. 14a and 14 d, PL II, illustrate the characteristic forms of the 
four-parted chromosomes as their portions are being separated preparatory 
to passing to the poles. These portions are split, and the halves diverge at 
the same time. In Fig. 15 a, PL II, which represents an anaphase stage of 
the division, the receding univalent chromosomes are distinctly two-parted, 
just as they are in Thalictrum purpurascens. Fig. 15 £, PL II, shows one of 
these receding chromosomes on a larger scale, with the two parts lying 
close together. Fig. 16, PL II, which is a drawing of one pole of the spindle, 
shows that the chromosomes approach each other very closely as they 
approach the pole. Only four of the twelve chromosomes appear in this 
section. The reduction division is thus completed. The reconstruction of 
the daughter nuclei follows rapidly. Fig. 17, PL II, represents a polar view 
of a daughter nucleus with the chromatin in as complete a condition of 
distribution as I have been able to find. In this figure the chromosomes 
are still visible, with the two portions lying parallel. No continuous 
chromatic spirem is formed. The phenomena of reconstruction of the 
daughter nucleus in Calycanthus Jloridus is identically the same as has 
already been described for Thalictrum purpurascens. Figs. 18 and 19, 
PL II, show both polar and side views of homeotypic division spindles, with 
the two-parted chromosomes arranged in the equatorial plate. Fig. 20, 
PI. II, is a drawing of a pollen grain, in which the nucleus may be seen to 
contain twelve prochromosomes as single unpaired structures. Each pro- 
chromosome is exactly the same size as each prochromosome in Fig. 1, 
PL II. In the first case these bodies are paired. In the pollen grain nucleus 
they are present as unpaired structures. The phenomena have been 
described for Thalictrum , and the prochromosomes are also present in the 
pollen grain, arranged into a single series. 
D 
