128 Mottier . — Mitosis in the Pollen Mother-cells of 
ance of the nucleolus. The nucleolus undoubtedly changes into a substance 
or substances that do not take chromatin stains, and I am further 
strengthened in the conviction, expressed in earlier papers, that the 
nucleolus not only supplies material for the chromatin, but also for other 
parts of the cell. 
On their way to the poles the daughter chromosomes undergo the 
longitudinal fission, or it may be that they merely reveal the longitudinal 
split which had taken place in the spireme stage (Fig. 42). As they reach 
the poles they are closely crowded together, so closely that they often 
appear as a fused mass. In polar view they are seen to form a rather 
compact disc. 
The construction of the daughter nucleus. The daughter chromosomes 
on approaching the pole are probably brought into close contact by the 
convergence of the spindle fibres, for it is very difficult to escape the con- 
clusion that the fibres are the active agents in the disposition and move- 
ments of the chromosomes. The assembled chromosomes now come to 
occupy the position of the spindle poles. When this has been accom- 
plished, they begin to separate gradually from each other. On the side of 
the mass towards the centre of the cell, the cytoplasm seems to withdraw, 
leaving a vacuoli-like space free from granules, and at the same time a sharp 
boundary between the colourless space and the cytoplasm is seen, which is 
continued around the mass of chromosomes (Fig. 43). This cavity in which 
the chromatin now lies is the nuclear cavity, and its boundary is the nuclear 
membrane of the daughter nucleus. 
As stated in the foregoing for Acer negundo , the chromatin collection 
separates into pieces irregular in outline, and connected by delicate anasto- 
mosing threads (Fig. 44). This figure is a polar view of a stage a little 
later than Fig. 43. Each large piece of chromatin in this figure represents 
probably more than one daughter segment, and the larger ones doubtless 
undergo further fragmentation, or alveolization into smaller pieces. As is 
well known, the extent of the alveolization or fragmentation varies in 
different plants, extending in some, as in Pinus , to the finely divided state 
known as the resting nucleus. Fig. 45 represents a longitudinal section of 
a daughter nucleus from a neighbouring cell in the same section as that 
from which Fig. 44 was taken. The pieces of chromatin are smoother and 
more slender. I did not find a continuous chromatin spireme in the 
daughter nuclei. The chromatin in the daughter nuclei of Staphylea does 
not reach the finely divided state of the resting nucleus. 
The details of the second mitosis are the same as those in Acer 
negundo , 
