Bristol . — Life-history and Cytology of Chlorochytrium grande . 1 1 3 
only 6-7 /x in thickness rapidly assumes a uniform indigo colour when 
treated with iodine. 
Within the cytoplasm there are also a variable number of pyrenoids ; 
these are brought out best in sections stained with Heidenhain’s haema- 
toxylin. In some cells there appear to be no pyrenoids at all, while in 
others as many as six large ones, or a considerable number of small ones, 
may be seen in a single section. In the larger pyrenoids the py reno- 
crystal is generally either polyhedral or spherical in shape, and there is 
a definite but rather narrow surrounding starch-sheath; smaller pyre- 
noids have usually spherical pyreno-crystals. The pyrenoids appear to 
multiply by fragmentation, the starch sheath gradually disappears, and 
the pyreno-crystal splits up into a number of small rounded bodies, each 
of which becomes the central mass of a new pyrenoid (Fig. 11). 
The resting nucleus, as seen in the vegetative cell, is relatively very 
large (PI. VI, Figs. 21-3). It has a definite nuclear membrane and is filled 
with a network differing very considerably from that of the surrounding 
cytoplasm. There is generally one very large, deeply staining, granular 
nucleolus, and a number of granules are present in the nuclear network, 
particularly at its angles. All evidence obtained from stained sections, 
especially from those stained with Heidenhain’s haematoxylin, shows that 
these granules are of an achromatic nature, and are probably similar in 
composition to the granules in the outer cytoplasmic network. The 
chromatin of the nucleus is entirely absent from the nuclear network, and 
is collected together to form the large nucleolus, which is therefore of 
the nature of a karyosome. The nucleolus is not homogeneous in structure, 
but contains frequently one or more less deeply staining areas. In a few 
cases peculiar conditions have been observed in which the nucleolus appeared 
to be filled with a number of vacuoles surrounded by intensely staining chro- 
matic substance (Figs. 22 and 23). These may merely have been indications 
of the incipient decomposition of an otherwise apparently healthy cell ; on 
the other hand, they may possibly have been preliminary stages in the 
breaking up of the nucleolus to form a chromatic thread preparatory to cell 
division. In one nucleus three karyosomes were observed, but this was the 
only cell found containing more than one nucleolus in the nucleus (Fig. 24). 
Multiplication by Means of Aplanospores. 
When the vegetative cells have reached a diameter of about 65- 
70 /x, changes begin to take place in the cell-contents preparatory to the 
formation of aplanospores. Numerous vacuoles appear in the cytoplasm, 
and after a time the whole of the cell-contents divide simultaneously to form 
a number of aplanospores (Fig. 31). These are naked granular protoplasts, 
each consisting of a nucleus and cytoplasm containing uniformly distributed 
chlorophyll. They are usually spherical and from 5*5 to 6-5 fx in diameter, 
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