Pollen Mother -cells of Li Hum canadense . 205 
and Gregoire (’ 99 ). Mottier’s figures of the chromatin bodies in the pollen 
mother-cells of L . Martagon and Helleborus foetidus are very similar 
to those which I have observed. 
Fig. 25 shows a tangential view of a portion of the longitudinally split 
spirem, in which the course of the threads is easily traceable. Fig. 2 6 
represents a thin median section of a nucleus in the same stage, in which 
the threads are frequently cut and occasionally displaced by the knife. In 
general the two threads are more or less tightly twisted about each other, 
in places so tightly that it is difficult or impossible to determine more than 
a single strand of double thickness ; in some places they are more widely 
divergent, and in others they run parallel for a distance without any 
twisting. 
The general course of the double thread and its distribution in the 
nuclear cavity are at this stage the same as before the fission. The 
external projections of the chromomeres, with the attached fibres, do not 
show in the figures, but in material more favourably stained they are plainly 
present. These projections appear on what was, before the splitting, the 
outer surface of the thread, the inner surface of the daughter-threads (i. e. 
the plane in which the splitting has occurred) being quite smooth and 
evenly stained. Very shortly after the completion of the longitudinal 
fission, there is manifest a tendency for the thread to become aggregated 
in the central portion of the nuclear cavity, leaving fewer loops which reach 
to the nuclear membrane. The nucleole shown in Fig. 26 is very irregular 
in outline. This, however, is exceptional. Somewhat elongated nucleoles 
are occasionally seen, but the great majority are nearly or quite spherical, 
and they are seldom vacuolated, differing in this respect from their 
appearance in earlier stages. 
Chromosome Formation. 
In nuclei in which the tendency toward a central aggregation of the 
spirem first becomes apparent there occasionally appear free ends of the 
thread ; as the spirem is double, these ends are always in pairs. They 
occur in sections thick enough to include a whole nucleus, so that the 
break in the thread cannot be due to cutting. The free ends observable 
are usually at or near the periphery of the nucleus. Sometimes two pairs 
of ends lie very close together, and in some such cases they can be seen 
to be connected by a very lightly-staining substance. It is evident that 
at these places the transverse segmentation of the double thread is occurring 
or has just occurred. The number of free ends, if any, to be distinguished 
in a nucleus at this time is very small ; and since, if segmentation were 
simultaneous in all parts of the thread, twenty-four pairs of ends would 
be present, it follows that the segmentation is successive. Whether any 
Q ^ 
