Trow . — The Karyology of Saprolegnia. 617 
ready for use. Flies killed by chloroform to prevent injury 
to their chitinous coats, moistened with alcohol and washed 
with sterilized water, are next thrown into the jars. The 
infected leg of the fly is then taken and placed in contact 
with one of the floating flies. In a week the flies are well 
covered with so-called pure cultures of the species selected. 
The cultures are not, however, pure, as they contain bacteria, 
monads, infusoria, &c., and in specially hot weather these 
unwelcome organisms affect more or less unfavourably the 
healthy development of the cultures. 
Successive generations may readily be produced by infecting 
a second jar with water from the first, a third with water from 
the second, and so on. To attain certainty as to the purity 
of a culture, it is well to keep it under microscopic control for 
several generations, and to adopt the usual precautions with 
respect to the sterilization of the various instruments used in 
the different operations. De Bary was able to maintain the 
purity of a culture of 5. Thureti for a period of eleven years. 
I have obtained a pure culture by placing with the leg of 
a fly, as described, a single sporangium in which the spores 
were already outlined. The spores germinated in situ , in- 
fected the leg, and produced the required cultures. Cuttings 
of the hyphae behave in the same way. 
On one occasion I succeeded in making a pure culture of 
Aphanomyces laevis from a single zoospore. The method 
adopted in this case may be of use later in determining the 
real nature of the diclinous species : it is certainly possible 
that these are dioecious. White of egg was beaten up and 
a small drop spread thinly on a cover-glass, which was then 
immersed in boiling water in order that coagulation might 
take place. The thin layer of albuminous matter is sufficiently 
transparent for observations to be made through it, and it 
provides suitable and abundant nourishment for the develop- 
ing plant. These covers are then floated on the surface of 
the water in the jars containing a mixture of species of Sapro- 
legnieae. Of ten covers floated on the surface of a jar, nine 
were found after a day or two to be unaffected, the tenth had 
