Bristol, — Life-history and Cytology of Chlorochytrium grande. 1 19 
where there is a clear spot, probably the nucleus. Exact details of the 
cytology of the zoogonidia could not be obtained on account of the rarity 
of their production, which prevented the use of staining methods. It was 
also found impossible, on the same account, to follow the nuclear changes 
which take place during the formation of zoogonidia. 
The zoogonidia swim about in the water for nearly an hour by means of 
their cilia ; it sometimes happens that two zoogonidia become entangled by 
their cilia, but fusion does not take place, and they finally come to rest, 
round themselves off and grow rapidly, developing directly into vegetative 
cells. For a short time the young developing zoogonidium has no cell-wall, 
but after a few days a thin cellulose membrane is formed. The characteristic 
lobed chloroplast is soon differentiated and development into a vegetative 
cell continues in exactly the same way as that of an aplanospore. The 
production of a red pigment has not been observed at any stage of the life- 
history of this alga. 
Affinities and Systematic Position. 
A comparison of the life-history of this form with that of Centrosphaera , 
Borzi, shows that undoubtedly the two algae belong to the same genus. In 
both cases the vegetative cell is globose or ellipsoid with a firm thin cell-wall. 
Upon reaching a certain size the contents of the cell divide simultaneously 
into a number of aplanospores which develop directly into vegetative 
cells. After an indefinite number of generations has been produced in this 
way the vegetative cell becomes converted into a hibernating zoogonidangium 
with a characteristically thickened cell-wall. From the zoogonidangium a 
large number of motile biciliate zoogonidia are produced. These come to 
rest and grow at once into vegetative cells, which, in their turn, multiply by 
means of aplanospores. 
There are, however, a number of differences which are worth noting. In 
the alga described above the average diameter of the vegetative cell before 
aplanospore-formation takes place is 65-70 /x, and of the zoogonidangium 
about 130 \x. The shape and thickening of the zoogonidangium-wall is very 
variable ; the internal projections may be short and conical, or long, twisted 
and even branched within the cytoplasm, and more than one external pectic 
projection may be present on a cell. The zoogonidia are produced by the 
successive bipartition of the contents of the mother-cell. 
In Centrosphaera Facciolae, Borzi, on the other hand, aplanospore- 
formation takes place when the vegetative cell has reached a diameter of 
only 30-40 fi , and the zoogonidangia are rarely more than 80 /x in breadth. 
The zoogonidangium - wall is much less variable ; it bears 1-3 small conical 
internal projections, and a single large external projection often curved into 
the form of a spur. Borzi describes the formation of zoogonidia as being due 
