Chromosomes in Pollen Mother -cells. 323 
central and entangled mass (Figs. 9, 10). During this second contraction 
or rearrangement the spirem becomes partly or wholly segmented into 
the chromosomes. In some cases the twisted loops radiate somewhat 
diagrammatically, but this regularity is only exceptional. Fig. 9 and 10 
which are slightly different stages, show average conditions met with in 
the many preparations. In Fig. 9 the spirem has not completely segmented, 
but in Fig. 10 segmentation seems to be complete or nearly so. In this 
and in nearly all other cases the entangled state of the chromosomes 
obscures their inner ends. In a great many instances the loops and other 
parts of the spirem are more entangled than is shown in these figures. 
From Figs. 8, 9, and 10 it is seen that the two segments, which are more 
or less twisted about each other, represent the bivalent chromosomes, and 
that each bivalent chromosome is formed by the coming together or 
approximation of two portions that were previously arranged end to end 
in the spirem. In Podophylhim there is no marked divergence of the 
longitudinal halves of the spirem during rearrangement, as may occur in 
Lilium. In the majority of cases this approximation of different parts of 
the spirem side by side is accomplished by a looping, but the segments 
may come together side by side, or adhere end to end after cross segmenta- 
tion, as will be seen especially in Tradescantia. Frequently the curved 
ends of the radiating loops are directed toward the nuclear membrane with 
the free ends at the centre, but this is not always the case, and it may not 
be the rule, for the free ends are often seen pointing towards the periphery 
(Fig. 10). The free ends of the chromosomes are almost invariably 
connected with the nuclear membrane by delicate fibres. It sometimes 
happens that the longitudinal split, which is seen in the spirem on coming 
out of synapsis and which disappears from sight in the hollow spirem, can 
be observed in each parallel part of a loop (Fig. 10). The writer under- 
stands that this longitudinal fission of the spirem persists, but from the 
stage of the hollow spirem to the meta- or anaphase the halves are so 
closely applied as to obscure the split. During the anaphase, or in the 
early telophase, the longitudinal fission again becomes evident, and re- 
presents the line of separation of the daughter segments in the second, or 
homotype mitosis. In my earlier paper this fission was regarded as 
a second longitudinal splitting (Mottier, ’ 03 ). 
Between the time of the segmentation of the spirem into chromosomes, 
which takes place either during or following the second contraction, and 
the formation of the spindle, they shorten rapidly and become very much 
thicker. This, and not during the hollow spirem stage, it may be added, 
is the period of greatest shortening and thickening of the chromosomes 
in all plants observed. An error made by the writer in common with 
almost all other observers was the assumption that the greatest shortening 
and thickening of the spirem took place before cross segmentation. 
