490 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The strength and rigidity of this sni^porting stalk is still further 
increased by the mucus deposited on its exterior by the colony of 
undifferentiated myxamoebae, or pseudoplasmodium, as it ascends 
the stalk ; and a mucous membrane, furthermore, binds the base of 
the stalk to the substratum. 
While the stalk in three members of the Dictyosteliaceae consists 
either of a single row of similar superposed cells or a more or less 
broad column of similar cells, in the fourth genus, Coenonia, the 
external cells of the column are still further differentiated by the 
formation of a j^apilla on the perij^heral wall of each, and the sum¬ 
mit of the stalk is expanded into a cupule. 
While both the sessile and the stalked sori usually consist of spheri¬ 
cal naked masses of spores or pseudospores, held together by means, 
of a mucous substance, in Acrasis, the spores are in a single row, or 
chain, surmounting a stalk which is likewise composed of a single 
row of superposed cells. 
In the fructifications of the Giittidinaceae occur two types of rest¬ 
ing bodies: in Guttulinopsis and in Sappinia, the individuals, dur¬ 
ing their resting condition, become encysted, secreting no definite 
wall, thus forming pseudospores ; while in Guttulina, the individuals 
form definitely walled spores. In the higher Dictyosteliaceae^ there 
invariably occur spores, which possess a single membrane composed 
of cellulose, augmented in the case of Acrasis by a cuticularized 
external zone. In germination, the wall of the spore is ruptured, 
while in the case of the pseudospore, the j^rocess is not accompanied 
by the breaking of a membrane, but the resting individuals simply 
gradually reassume a state of activity. 
In the individuals of Dictyostelium and Polysphondylium, occur 
what appear to be two types of division, one in''the young individ¬ 
uals shortly after germination, and the other during the subsequent 
active amoeboid condition. The first, or primary division occupies 
a number of hours and is accompanied by prolonged nuclear changes 
which resemble karyokinetic division. The secondary division re¬ 
sembles rather direct division and takes place usually within a few 
minutes. 
The force which is operative in inducing the,fructification to be 
directed away from the substratum is evidently negative hydrotro¬ 
pism. While gravity does not seem to affect the direction of the 
sporophores, the fructifications, at least of the higher forms, are 
