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MALAY FORM OF CHLOROCOCCUM HUMICOLA, 477 
sometimes pointed at both ends. Each has a single bell-shaped chloroplast 
fitted against the posterior end of the zoogonidium, though in the case of a 
few elongated zoogonidia the posterior end was found to be colourless, the 
chloroplast being in the form of a parietal band round the middle of the cell. 
There is a red pigment-spot near the anterior end of the cell and a single 
pyrenoid in the chloroplast. In some cases, owing to cultural conditions, 
the escape of the zoogonidia was observed before the division of the mother- 
cell-contents was complete, and a number of extremely abnormal zoogonidia 
were formed ; one of these is shown in fig. 14 4. with three pairs of cilia, 
three pigment-spots, and three distinet chloroplasts. 
When the zoogonidia have made their escape from the mother-cell, they 
swim about for a short time and then frequently fuse in pairs, either with 
one another or with zoogonidia from another mother-cell. Two zoogonidia 
become entangled by their cilia and fusion takes place from the anterior end 
in the usual way to form a zygote having four cilia attached at the anterior 
end. One case, however, was observed in which the fusion took place in 
such a way as to produce a zygote with two cilia at each end, while in 
another a normal zygote was seen with a small zoogonidium partially fused 
to its posterior end. As a general rule, the zoogonidia were of the same 
size, but a few cases were observed in whieh fusion took place between 
anisogametes. 
A fusion of this kind does not seem to be essential to the life-history 
of the alga, for a great many zoogonidia, after swimming about for some 
time, come to rest and develop in exactly the same way as the zygotes. The 
cells round off, lose their cilia, and, acquiring cellulose cell-walls, develop by 
gradual stages into adult vegetative cells. 
V. MULTIPLICATION BY APLANOSPORES. 
In certain circumstances a vegetative cell produces non-motile gonidia 
instead of zoogonidia. The nuclei of the mother-cell appear to increase 
considerably in size and number, and each becomes the centre of a little 
mass of cytoplasm which forms an aplanospore. A great many are produced 
from a single mother-cell and they become angular by compression. In 
stained sections of cells about to form aplanospores the nuclei are very 
conspicuous and the nuclear space is seen to be finely granular with an 
occasional larger granule of chromatin. The karyosome is almost central in 
position, and may be spherical or oval, or somewhat elongated with a slight 
constriction in the middle. In these cells the nuclei have been observed to 
divide by constriction (fig. 21), and it may be that the irregularities in shape 
of the karyosome are preliminary stages in the division of the nucleus. The 
occurrence of this form of nuclear division in Chlorococeum is surprising in 
view of the fact that mitotic figures have been obtained for both of the allied 
