Acton. — Studies on Nuclear Division in Desmids. I. 381 
Division of Nucleus and Cell. 
The first sign of division in the nucleus is the appearance of granules on 
the nuclear network, which consequently shows up more clearly on staining 
(Fig. 3). At the same time the nucleolus stains less deeply and finally 
disintegrates. The small size of the nucleus makes it exceedingly difficult 
to follow exactly what happens in the succeeding stages. The granules 
appear to increase in size and decrease in number, so that probably the 
chromosomes are being formed on the spireme. The gradual disappearance 
and disintegration of the nucleolus makes it improbable that the chromo- 
somes come bodily out of the nucleolus. 
The next stage that could be definitely seen was the collection of the 
chromosomes on the equatorial plate (PI. VIII, Fig. 6). The chromosomes 
are short broad rods, almost granules, and about twelve in number. A 
definite spindle could not be seen, but fibres could clearly be seen attached 
to the chromosomes and pulling the daughter chromosomes to the opposite 
poles of the nucleus (Figs. 7 and 8). 
In the reconstruction of the daughter nuclei the chromosomes lose their 
identity, granules appear in the nucleus, and these gradually fuse to form 
the large nucleolus (Figs. 9-12). 
The daughter nuclei begin to separate immediately they are formed, 
and to move in opposite directions towards the surface of the cell, where they 
take up a position opposite the pyrenoid. 
Formation of the new cell-wall was not observed in detail. It is 
always completed before the division of the chromatophore begins, so that 
for a time the cell has only one chromatophore. 
As the chromatophore divides the nucleus slips in between the two 
halves until it finally reaches the pyrenoid and, during the division of the 
pyrenoid by constriction, remains firmly pressed up against it. The division 
of the chromatophore and pyrenoid is probably largely influenced by 
the presence of the nucleus. No trace of division appeared in the chromato- 
phore until the nucleus had moved round to the surface of the cell and 
taken up its position opposite the pyrenoid. Constriction does not take 
place equally all round the chromatophore, division always being further 
advanced in the part in which the nucleus lies (see Text-figs. 1-4, p. 382). 
The division of the pyrenoid and chromatophore has already been 
figured and described by Schmitz, 1 but the close connexion between the 
division and the presence of the nucleus was not noted. 
The starch-sheath of the pyrenoid seems to be fairly intact at the 
completion of the division of the nucleus, though signs of disintegration 
have already appeared. It is not until the beginning of division in the 
chromatophore that it decreases rapidly in size, and at the end of this 
process no trace of the starch-sheath is visible. 
1 Schmitz, F. s Die Chromatophoren der Algen. Verhandl. d. naturf. Ver, Bonn, Bd. xl, 1880 
