220 Barrett . — Development and Sexuality of some 
far would inevitably prevent the desired results. Not all cultures with this 
species gave results, and the successful ones needed to be carefully watched 
in order that their productiveness be maintained. After the sexual spores 
had attained the desired stage of development most of the larvae were 
transferred to other vessels in order that new ones could be added. It was 
necessary, at times, to deviate in one way or another from the regular 
method described, in order to overcome one or more difficulties. 
The sexual spores of O. Saprolegniae were as difficult to obtain in 
quantity as those of O. vexans. Those of O. luxurious were secured in by 
far the greatest numbers and with the least difficulty. The host, Aphano- 
myces laevis, was grown on green aphids, a supply of which was kept sterile 
in test-tubes. The value of the aphids as culture media was not discovered 
until most of the cultures of the other species were brought to a close, but 
the few trials made showed that the aphids were equally serviceable for 
them. They furnish a very limited amount of nourishment, in that one can 
find them of all sizes, which fact seems to bear a direct and favourable 
influence on sexual spore production of the parasite. Correlated with this 
factor of a small amount of nourishment is the limited amount of water, 
which should remain unchanged. One disadvantage in the use of small bits 
of substrata is the reduced size of the sexual spores of the parasite that 
results. Another advantage obtains in the use of this method. Having 
the zoospores of both host and parasite in the culture provides for the 
immediate infection of the host hyphae as they emerge from the substrata. 
This enables the parasite to get a good start before the hyphae reach any 
length, and prevents the formation of a tangled mass of hyphae, so ob- 
jectionable in the preparation of the material for cytological study. The 
result is a dense halo of short swollen filaments containing the parasites in 
all stages of development. In the case of the two species on Sapro- 
legnia it is really necessary that infection take place when the hyphae are 
young. 
So far as my observations with the three species of Olpidiopsis go, 
there seems to be no foundation for a view that a production of sexual 
spores need be preceded by a somewhat continuous production of sporangia, 
or that the two need alternate with each other. On the other hand, if the 
proper external conditions are provided, such spores are more or less easily 
obtained. 
It should be stated here that the above-described method also provided 
good material of the sporangia in the resting stage. 
The sexual spores develop in the same position as the sporangia, with 
which they may or may not be associated. Usually, however, one finds 
one or more sporangia in the same hyphal swelling containing sexual spores. 
The greater the degree of infection of the host, the greater the varia- 
tion in the size of both kinds of reproductive bodies. In general, the manner 
