Bristol ' — Life-history and Cytology of Chlorochytrium grande . 1 1 5 
rest. Stained sections of young vegetative cells, of diameter about 18 /x, 
show an exceedingly simple structure (Fig. 34). There is a more or less 
central nucleus and a simple chloroplast almost filling the cell, with its outer 
surface raised into a few rounded lobes ; the colourless cytoplasm filling the 
spaces between the lobes is sometimes very evident, and there may be one or 
two pyrenoids present. As the cells increase in size the cytoplasmic 
reticulum becomes much more marked, and the lobes of the chloroplast in- 
crease in number until the cytoplasm assumes the structure shown in Fig. 6 . 
The aplanospores produced by a single mother-cell do not all necessarily 
develop at the same rate ; clusters of young cells are often seen in the con- 
dition shown in Fig. 33, where one of the cells has attained a diameter 
of 56 /x, and several others of about 22/*, while a number of the aplano- 
spores have scarcely begun to develop. 
When the cell has developed to its full size a repetition of the fore- 
going processes takes place, several successive generations of vegetative 
cells being produced by means of aplanospores before any further changes 
take place. 
Reproduction by Zoogonidia. 
After an indefinite number of generations of aplanospore-formation 
the vegetative cell undergoes important changes, being gradually trans- 
formed into a zoogonidangium. This may occur at any stage of growth 
of the vegetative cell, but usually takes place after the cell has attained 
its full size ; and it is to be noted that propagation by aplanospores has 
been observed after transformation into a zoogonidangium has begun. 
The cell-wall becomes enormously thickened in both layers and shows 
numerous stratifications. The cellulose layer may become 4-8 /x or rarely 
as much as 11 /x thick, while the pectic layer varies from 2 to 7 fx in thickness ; 
the whole thickness of the wall varies usually from 6 to 15 /x, but specimens 
have been found with their walls both above and below these limits. The 
thickening is not distributed evenly over the whole surface of the wall, but 
in both cellulose and pectic layers it may be concentrated to form one 
or more projections. Unequal thickenings of the cellulose layer give rise 
to internal projections of very varied sizes and shapes, which show all the 
characteristic reactions of cellulose. They may be in the form of small 
conical papillae showing fine transverse striations, as observed by Borzi, and 
where they are of this form there are usually several in one cell. Fre- 
quently, however, they assume a very much larger size, projecting more 
than half-way across the cavity of the cell. These long projections may be 
simple with an enlarged globose or irregular head (Text-fig. 1, j), or they 
may be branched and twisted, ramifying through the substance of the cyto- 
plasm (PI. V, Fig. 1 5). As a rule only one such projection is formed within 
a single cell, but two have been observed (Fig. 13), and it sometimes 
