OLIVE : MONOGRAPH OF THE ACRASIEAE. 
475 
with that seen in the Dictyosteliaceae. In the latter group, luxu¬ 
riant growths of Dictyostelium and Polysphondylium may result in 
fructifications a centimeter or more in length; in some species, on 
the other hand, they normally attain a height of only one to two 
millimeters. The simplest type of fructification in this higher group 
is evidently that of Acrasis, in which a single row of amoebae 
becomes converted into a stalk terminated by a single chain of spores. 
The fructifications of the remaining three genera of the group, on 
the other hand, furnish examples of increasing complexity of struc¬ 
ture ; that of Dictyostelium, for example, is usually simple and com¬ 
posed of a stalk bearing a spherical sorus of spores at the summit, 
thus resembling fructifications of Mucor. That of Polysphondylium 
is normally a racemosely branching fructification in which the 
branches, as well as the main axis, are each terminated by a spherical, 
naked mass; while the greatest degree of differentiation is presented 
by Coenonia, in the fructifications of which, according to van 
Tieghem, each peripheral cell of the axis bears an external papilla or 
tooth, and the summit of the stalk is expanded into a cupule, with a 
dentate border, in which is supported the spherical sorus of spores. 
The following account of the stalk formation in the Dictyostelia- 
ceae is based upon a detailed study of the development of the fructi¬ 
fications of Dictyostelium and Polysphondylium only. In all 
probability, however, the development of Acrasis and Coenonia, 
concerning which few details are given by van* Tieghem, conforms 
in general with the conditions to be described below, since the prin¬ 
cipal phenomena connected with the fructifications in all four forms 
appear to be similar. 
The first visible indication of differentiation in form and function 
in the originally similar myxamoebae of the pseudoplasmodium, con¬ 
sists in the formation of one or more permanent vacuoles in certain 
centrally jdaced individuals (pi. 8, fig. 110). It is possible that the 
contractile vacuoles present in the undifferentiated individuals 
throughout their active upward movement, may become converted 
into these permanent vacuoles by the production of a colloidal mem¬ 
brane about each individual destined to take part in stalk formation, 
although I have so far been unable to determine this point definitely. 
This peculiar vacuolation occurs at first in certain individuals in con¬ 
tact with the substratum; in a drop culture, such a group of myx¬ 
amoebae is readily distinguished, since those which are thus differ- 
