1 1 8 Mot tier. — Mitosis in the Pollen Mother-cells of 
other plants, the writer has found much variation in the time at which the 
first rounding off of the mother- cell is distinguishable, using the appearance 
of the synaptic mass as a guide. In the earliest stages of the contraction 
the cells are invariably polygonal, forming a compact tissue without any 
intercellular spaces (Figs. 7, 8). With the appearance shown in Fig. 8 the 
rounding off usually begins, and the same is evident in all cells as soon as 
the contracted ball has loosened up sufficiently to show clearly the definite 
spireme character (Fig. 11). Whether the appearance of the first rounding 
off of the cells is due to a slight shrinkage caused by the reagents cannot be 
stated with certainty. It is possible, of course, that this may be true to 
a limited extent, for it not infrequently happens that the cells in one loculus 
may be noticeably or even badly shrunken, while those of the neighbouring 
loculus of the same anther will show perfect fixation. 
As pointed out for other plants, the loosening of the synaptic mass is 
first manifested by the extension of loops or turns of the spireme into the 
nuclear cavity (Fig. 11). These continue to spread out, and soon the entire 
chromatin cord is distributed throughout the nucleus, forming what is so 
well known as the hollow spireme. The larger portion of the spireme is 
probably arranged along the nuclear membrane, yet it also traverses the 
cavity in various directions, some portions following straight courses 
through the cavity, others being more or less looped or kinked. Fig. 12 
represents the stage of the thick hollow spireme in section, not all of the 
nucleus being shown. The earlier steps in the formation of this spireme 
have shown that it is made up of a succession of chromomeres that are held 
in the linin. No longitudinal fission or double nature of the spireme is seen 
in this plant, although this fact is not a proof that a longitudinal fission 
may not have taken place, as is the general rule in plants, and as will be 
seen in Staphylea , to be described below. The fact that the spireme is 
seen as a single cord makes it easier to trace certain later steps in the 
mitotic process with much greater certainty than were the confusing feature 
of the diverging halves of the spireme present in longer or shorter stretches, 
as frequently happens in the Lilies and in other plants. At this stage the 
pollen mother-cell has separated from the primary cell-wall and has formed 
a soft, thick wall of its own. The nucleolus is still present, and it is about 
as large as in any preceding stage. 
The formation of the bivalent chromosomes from the hollow spireme . 
The next series of steps in the process following the regular, thick, hollow 
spireme result in the formation of the bivalents from the same. This tran- 
sition brings about certain changes by which the regularity in the disposition 
of the turns of the spireme is lost, and which give rise to the phenomenon 
known as the second contraction. In the second contraction the spireme 
is so rearranged that a part of it is more or less closely entangled into 
a knot near the centre of the nuclear cavity, from which extend somewhat 
