620 The American Naturalist. — [August, 
stated, that plasmodia and Vorticellidee were watched for peri- 
ods which varied from one to fifteen days; large plasmodia 
were isolated or preserved in concave slides. Even on plane 
slides the pressure of the cover slip was slight enough to allow 
of the emission of short pseudopodia in planes at right angles 
to the plane of extension of the slide and the animals, by 
means of pipettes, were transferred to fresh water daily. 
In a synopsis of the work of Greenwood and Saunders, in a 
previous report bearing on this matter, in the Journ. of 
Physiol., the changes undergone by litmus, Congo red and 
alizarin sulphate, and the solution of the globoids of aleurone 
grains, which are composed of a delicate nitrogenous capsule 
enclosing pure calcium and magnesium phosphate, were de- 
scribed. It was emphasized that the outpouring of acid is un- 
accompanied by any digestive change on nutritive matter ; 
ingesta may indeed be stored for many hours in vacuoles be- 
fore they are dissolved, or digestion may follow rapidly on in- 
gestion. But the formation of the digestive vacuole, whether 
immediate or delayed, is preceded by the development of acid 
reaction and followed by its diminution. Bearing in mind 
that litmus is changed from blue to red not only by free acid, 
but also by unsaturated compounds of acid with the products 
of digestion, i. e., acid salts. And that Congo red changes to 
blue in presence of free acid only. Itis apparent that the dim- 
inution of acid in a digestive vacuole is at first due to a com- 
bination with the products of digestion, for at this stage any 
litmus accompanying ingesta is still red, while Congo red has 
reverted to that tint from blue. Here free acid is absent but 
acid salts are present. But later on the vacuoles and ingesta, 
reddened by litmus, become violet and blue so that finally 
acid and acid combinations are alike absent. That the acid 
is at one time free is indicated unmistakably by the striking 
development of violet colors in solids stained with Congo red. 
Now as the amount of acid present at any moment must be 
very small, and this being so, that the change in Congo red | 
should be speedy and striking suggests that it is an inorganic 
acid but it is probable that to emphasize such an inference 
would be hasty. 
