468 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
separated from ttie outer covering by the vacuolar fluid (pi. 5, fig. 
36-39). A second inner membrane may be added in the same way. 
In an impure decoction, the outer wall of the microcyst becomes 
thickened and incrusted with foreign particles, while in one that is 
pure, it remains unthickened and colorless. 
Van Tieghem has further observed a peculiar method of encyst- 
ment in Acrasis and in Dictyostelium, in which the myxamoeba 
pushes out a sort of budding process which forms a protective mem¬ 
brane about itself and finally becomes detached. This formation of 
microcysts by budding he saw taking place at several points at once, 
until at last all the protoplasm of the myxamoeba was converted 
into encysted buds. In Guttidinopsis vulgaris also, I have seen 
a curious phenonfenon, possibly comparable to the encystment 
described by van Tieghem, in which a number of small, rounded 
pieces were successively pinched off from the individual, although I 
cannot state with certainty that the detached buds were resting 
microcysts. 
Aggregation of Myxamoebae. 
It is only after a more or less prolonged independent existence as 
vegetating individuals, the time being dependent to some extent on 
the amount of moisture and food available, that the myxamoebae 
tend to congregate toward certain centers and form aggregations 
for the purpose of fructification. Although this period of colony 
formation is properly connected with the phenomena of fructification, 
as distinguished fi-om those of vegetation', it may properly be con¬ 
sidered apart from either. 
This remarkable phenomenon which involves the aggregation of 
great nuihbers of individuals to form the so called pseudoplasmo- 
dium, in which, though closely adherent, the myxamoebae remain 
mechanically separable from one another, is a distinctive character¬ 
istic of the Acrasieae. In all members of this order with one pos¬ 
sible exception, certain individuals, at this period, as a result of what 
may be assumed to be the secretion of some definite substance, 
become centers of aggregation, the attractive influence of which 
extends to a considerable distance. Sappinia, however, appears to be 
exceptional in this respect, since a chemotactic stimulus which could 
