Brown.—Studies in the Physiology of Parasitism. I. 323 
The spore material is extracted by suspension in distilled water, care 
being taken to keep the debris in suspension by shaking at intervals. By 
subsidiary experiments (using the quantitative method to be described 
below) it was shown that by increasing the amount of powder suspended in 
a given volume of water, the activity of the resultant extract increased, but 
only up to a certain point, further increases in the proportion of powder 
giving no appreciable increase in the activity of the extract. This limiting 
activity of extract is obtained when extraction is made in the proportion of 
°*2 gr. powder (= o*i gr. fungus+ o*i gr. sand) to 3-4 c.c. water. Thus 
the difference between the activity of, say, a o-2 gr. in 2 c.c. and a 0-2 gr. in 
3 c.c. suspension is, with powder of normal quality, scarcely demonstrable. 
In view of these considerations, the spore powder has been extracted 
throughout in the proportion of 0-2 gr. to 3 c.c. water, the full activity for 
the powder being thus obtained. 
In view of the statement of Michaelis (Abderhalden’s Handbuch d. 
biochem. Arbeitsmethoden III, i, p. 13) that in such cases, where the enzyme 
or similar substance is contained on the surface of an insoluble powder, an 
extraction of twenty-four hours’ duration is required in order to ensure 
uniformity of extract in different experiments, a series of experiments was 
set up to determine what time of extraction is necessary in order that the 
extract may reach its limiting strength. Extractions were made for periods 
of J, •!, f, and 1 hour, the process being stopped at the end of each period 
by centrifuging off the debris. These extracts were compared by the 
quantitative method to be described later. In the case of the ^-hour 
extract only was there any indication that the full strength had not been 
reached. The three other extracts were of equal strength within the limits 
of experimental error. Throughout this work, one hour’s extraction has 
been allowed, this time being chosen as representing safety as well as 
offering conveniences for the carrying out of the other experimental details 
involved. The liquid is finally cleared by centrifuging for three minutes at 
the highest speed available (3,000 revs, per minute), decanted, and again 
centrifuged and decanted. The liquid thus obtained is the crude extract 
which has been employed in the present investigation. It possesses a pale 
straw-colour, is opalescent, and has a characteristic ‘ mouldy ’ smell. 
Though the experimental method as sketched above seems somewhat 
laborious, it is by no means unduly so, and it is quite practicable to prepare 
in this way quantities of material comparable to those which are usually 
employed in enzymic studies. The following figures will suffice to show 
how the method works out, representing as they do a fair average: 
From two Petri dishes of 8 cm. diameter are obtained 0*5 c.c. spores 
(wet volume) ; the latter are sown in 50 c.c. Turnip extract on 10 plates. 
The weight of germinated material when dried = 0-7 to o-8 gr. Thus the 
powder obtained = 1*4 to 1 -6 gr. For the preparation of standard extract, 
