138 Trow . — Observations on the Biology and 
Observations, Chiefly Biological, made on Living 
Material. 
The normal course of development, from the germination 
of the zoospores to the maturation of the oospores, is com- 
pleted in from ten to fifteen days. 
If house-flies are thrown into a glass jar of one litre capacity, 
which has been nearly filled with water containing numerous 
zoospores, the following order of events can be easily followed 
with the naked eye. The mycelium may be detected as 
a fine halo around the flies at the end of the first period 
of twenty-four hours ; at the end of the second day the 
mycelium is well developed ; at the end of the third day 
it is full grown, and the first sporangia have made their 
appearance at the ends of the stoutest hyphae ; at the end 
of the fourth day the oogonia are to be seen in various 
stages of development as lateral outgrowths of the main 
hyphae. The culture shows no further perceptible growth, 
but the number of oogonia increases from day to day, 
the majority, however, being present at the end of the 
fifth day. Fig. 1 illustrates its condition at this time, and 
the habit of growth as well. Microscopic examination reveals 
the fact that oogonia with fertilized eggs are abundant at the 
end of the fifth day, and that the first crop of ripe oospores 
appears on the tenth. A culture fifteen days old is made 
up of an exhausted mycelium and an abundant crop of 
oogonia with ripe oospores. Under favourable conditions 
the oospores may germinate at once. In most of the 
cultures at the end of three months a few ripe oospores still 
remain associated with the empty shells of those which have 
germinated and produced zoospores. 
Such simple observations on the course of development are 
extremely important, as they do away, in most cases, with 
the necessity for a microscopic examination of the plants 
which it is proposed to fix. In order to obtain excellent 
material for the cytological study of the gametangia and 
