624 The American Naturalist. [August, 
sare secretes some zymogen which perfects the digestive secre- 
tion. 
The object to which the acid would seemingly serve in these 
organisms, which may be said to be on the very threshhold of 
life is the same which Bunge ascribes to it in man. Bunge’s 
view is that the HCl has no other purpose than the steriliza- 
tion of food. “ Why should a chemical substance be placed in 
the entrance to the digestive tract,” he asks, “in exactly the 
strength necessary for the destruction of bacteria which is di- 
rectly antagonistic to the chemical reaction in which the main 
work of digestion must be carried on? The proteids are more 
readily converted into a solution lower down in the intestine 
and in an alkaline medium than by pepsin and acid. The 
object of the acid is, according to him, then, one of steriliza- 
tion. This view cannot be denied, at the same time it 
must be admitted that HCl serves also a digestive purpose. 
In the Rhizopods experimented upon, the observations of 
Greenwood and Saunders could be confirmed concerning the 
fact that while the acid is secreted in the food vacuoles under 
the stimulus of all ingesta; the true digestive vacuole which 
occurs only under the stimulus of nutritive matter apparently 
` contains something besides an acid, perhaps an enzyme. The 
change in the acid indicators is as regards time and intensity 
of color transformation to all observation alike. There seems 
to be the same amount of acid in a storage vacuole as in a 
vacuole causing active solution of proteid matter, in close 
proximity to it, hence the assumption of an additional zymo- 
genic substance in the latter is justifiable. As the amount of 
acid in one of these vacuoles is very small, and the change in 
Congo red to blue is speedy and striking, lends belief to the 
suggestion of Greenwood that the acid is an inorganic one. 
Why the protoplasm around a storage vacuole will not secrete 
zymogenie matter, though acid is clearly present in it, and at 
the same time this enzyme must be accepted to be present in 
a vacuole in which, close to the former, active digestion is 
going on is a question difficult to approach. If it can be dem- 
onstrated that all or most storage vacuoles contain some sub- 
stance, living or inert, which is hostile to the economy of the 
Rhizopod and against which it protects itself by intensely acid 
