4g 6 Brown.— Studies in the Physiology of Parasitism . IV. 
on general considerations that the ascending part of the curve would also 
be found on the first day, but no experimental verification was attempted 
on account of the laborious processes which the preparation of the necessary 
amount of material would entail. 
Enzymic solutions of this type are characterized by the presence of 
a medium amount of enzyme. They contain a negligible amount of retard- 
ing substances and of crystalloids generally. 
The following practical rules may therefore be framed as bearing on 
the technique of preparing solutions of the enzyme under consideration : 
(a) In order to obtain a very active enzymic solution from the nutrient 
medium, it is essential that simultaneous development of a large number of 
spores take place throughout the whole nutrient medium. Where a small 
number of spores develop on the surface of a relatively large volume of 
nutrient, the enzymic activity of the latter never reaches a high value. 
(b) In order to obtain a strong mycelial extract, the germinated spores 
must be extracted while still young (one to two days old). Conditions of 
yield and of economy of space and labour necessitate as thick sowing of the 
spores as is consistent with their vigorous germination. 
In the case of both types of extract, the maximum activity is attained 
at a time roughly coincident with that at which vigorous growth of the 
hyphae ceases. 
The above results throw light on the relative merits of the extract 
employed in these studies and those employed by certain other investi- 
gators in the examination of the nature of the active principle concerned 
in the process of parasitic attack. The extract employed by the present 
writer shows the maximal activity obtainable from mycelial extracts, and is 
at the same time comparatively free from crystalloidal contamination such 
as occurs in the equally active preparations derived from the nutrient in 
which the fungus has developed. In Marshall Wards experiments , 1 extrac- 
tion of the hyphae took place too late (after three weeks’ growth) to allow 
of a strong extract being obtained. The extracts of de Bary and Behrens 
were obtained from the nutrient medium and contained therefore crystal- 
loidal contamination. It seems certain that the lethal effects of boiled 
extract in Behrens’s experiments were due to the large amount of crystal- 
loidal substances present, while the great age (six months) of his cultures 
precluded the presence of anything but the smallest traces of enzyme in his 
unboiled extracts, 
1 References to the papers cited here are given in No. i of the series. 
