302 
MAYNARD M. METCALF 
Careful observation of the contractile vacuole shows that it is 
surrounded by a layer of granules of the same size and appearance 
as the microsomes of the general cytoplasm (fig. 1). When the 
vacuole is of moderate size these granules form a continuous 
layer, one granule thick, the adjacent granules nearly if not actu- 
ally touching one another. When the excretory vacuole becomes 
very large the number of granules is insufficient to form a con- 
tinuous layer, the layer of investing granules becoming irregu- 
larly interrupted, as one readily sees by focusing upon the sur- 
face of the vacuole (fig. 2). When the vacuole is small it lies in 
a mass of granules which surround it several deep on all sides 
(fig. 9). 
Upon the contraction of the vacuole the granules formerly sur- 
rounding it remain, forming a very noticeable mass very different 
in appearance from the general cytoplasm (fig. 7). Tn the general 
cytoplasm the microsomes are loosely scattered. In the region 
from which the contractile vacuole has just disappeared in sys- 
tole, the granules are as closely crowded as they can lie. 
It is possible to keep this mass of granules under continuous 
observation for hours, and by so doing one sees that the contrac- 
tile vacuole, when it reappears, appears in the midst of this same 
mass of granules, and that this is continuously the case. I have, 
in a number of instances, observed the conditions continuously 
for from two hours to three and three-quarters hours, and in no 
instance have I seen an excretory vacuole reappear elsewhere 
than among the same granules which surrounded it before its 
last contraction. There is, therefore, a constant relation between 
the excretory vacuole and a group of granules which surround it. 
The details of systole and diastole are of some interest. Refer- 
ence to the accompanying figures will show, without lengthy de- 
scription, that after the vacuole has reached the full size, it crowds 
its way through the whole thickness of the ectosarc until at last 
it is separated from the circumnatant water by only the pellicle 
(figs. 1, 3, 4, 5, 16, 17 and 18). This pellicle is soon forced out- 
ward into a considerable protuberance (fig. 4) . Evidently it is 
quite tough and able to withstand considerable pressure. As 
the vacuole approaches the pellicle its granules first come into 
