Species of Olpidiopsis , (Cornu) Fischer . 219 
a number of exit tubes had been produced. Several of the tubes show 
a peculiar coiled condition of the end, while one is branched. This latter 
condition, which seems rare, was also observed once by Fischer in another 
species. Other sporangia in a similar stage, when given the proper con- 
ditions, failed to discharge zoospores. 
The exit tube secretes an enzyme which enables it to penetrate the wall 
of the host. In many cases, at least, mechanical pressure seems to play an 
important part. I have several times observed that the host wall was 
stretched out over the end of the protruding tube even when apparently 
some little distance through it. Very commonly, as has been previously 
observed, there is an enlargement of the tube just inside the hyphal wall. 
The escape of the zoospores takes place by the gelatinization and dissolution 
of the end of the exit tube. 
E. Development of Sexual Spores. 
The frequency of occurrence of the sexual spores of the organisms 
under consideration varies with the species and the condition of the cultures 
under which it develops. It has been assumed, in the case of resting spores 
belonging to species of a closely related genus, that they were formed only 
after a somewhat prolonged production of the zoosporangia, and then for 
the purpose of tiding the parasite over unfavourable conditions such as 
winter. They have been rarely found in the species of Olpidiopsis infecting 
members of the Saprolegniaceae, which fact, no doubt, partially accounts 
for our incomplete knowledge concerning their development. 
The first of these so-called resting spores found by the writer appeared 
in a culture of O. vexans on Saprolegnia ferax growing on ant larvae. 
Several cultures had previously been made on sterile flies with the result 
that only smooth sporangia developed, and it was, therefore, impossible to 
classify the organism. Three days after starting the culture above mentioned, 
sexual spores were numerous in all stages of development. New cultures 
were immediately started with a view to securing material for cytological 
studies. After making several series of cultures, I was somewhat surprised 
that the desired spores failed to appear. Almost a month of culture ex- 
periments finally revealed the requisite condition. Instead of starting new 
cultures in separate dishes, as I had previously been doing, I kept the old 
ones, to which at certain periods I added a new supply of ant pupae. These 
were disturbed as little as possible, and the water remained unchanged. 
There were always a sufficient number of zoospores of both host and parasite 
present to start a ready infection of the new substrata. 
By this method the otherwise rapid and vigorous growth of the 
Saprolegnia was somewhat retarded, and a large number of sexual spores of 
the parasite developed. It was detected that some contamination by 
Bacteria, protozoa, &c., seemed to be an advantage, but when carried too 
