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Beer. — Studies in Spore Development . 
which form a single layer round the mother-cells, are radially elongated 
structures which in the majority of cases enclose two nuclei, although cells 
with three or even four nuclei are met with. The stages of the division of 
the pollen mother-cells were but poorly fixed in my preparations, and, 
with the exception of the telophase of the second meiotic division, which 
was well shown in my sections, I will make no reference to the subject. 
At the conclusion of the second division of the pollen mother-cells the 
chromosomes retain their individuality for some time after a nuclear wall 
has been reconstructed and a new nucleolus (or nucleoli) formed in each 
daughter nucleus (PI. XIII, Fig. i a). The chromosomes are distributed 
throughout the nuclear cavity, but are connected with one another by a 
delicate linin threadwork. A little later the sharply defined, homogeneous 
chromosomes become more irregular in outline and apparently vacuolar 
in structure (Fig. i b ), and it is easy to trace the gradual opening out of 
their substance and its distribution over the linin network until no trace of 
any individual chromosome can any longer be detected (Fig. i c). After 
the division of these nuclei is finally completed, therefore, nothing can be 
seen in the nature of the ‘ prochromosomes ’ which Rosenberg (14, 15), 
Overton (12, 13), Laibach (10), and others have described in the ‘resting’ 
nuclei of various plants. 
I have examined the nuclei of the other tissues of the anther for 
prochromosomes, and in some, notably those of the young vascular tissue, 
chromatic aggregates are to be seen lying beneath the nuclear wall which 
resemble in appearance the prochromosomes of other writers (Fig. i d). 
Their number, however, appears to me to be too inconstant in these cells to 
have the significance which attaches to true prochromosomes. 
There is evidently great variation in the behaviour of the chromosomes 
of different cells at the conclusion of nuclear division. In some, such as in 
the cases described by Rosenberg and others, the chromosomes appear 
to retain a large proportion of their material definitely aggregated as clearly 
distinguishable prochromosomes throughout a prolonged period of rest. In 
other cases, such as the pollen mother-cells of Ipomoea , the chromosomes 
retain their individuality as distinct bodies for a short, but yet quite definite, 
period, and then their substance becomes evenly distributed over the linin 
reticulum. Finally, in many other cells the chromosomes become vacuolated 
and their substance dispersed over the linin threadwork immediately at the 
conclusion of mitosis. In cells such as those of the young vascular tissue of 
Ipomoea , in which an inconstant number of chromatic aggregates occurs, 
it is not improbable that some of the chromosomes may remain visible 
as distinct bodies for a greater or less time, others may become distributed 
over the linin at once and lose their visible individuality, whilst others may 
become vacuolated and broken up into two or more smaller but still 
recognizable bodies. 
