470 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
the same spot of a nutrient agar tube, will result in fructifications 
showing the two distinct forms growing side by side without 2 Lnj 
trace of intermixture. Myxamoebae which resemble each other 
exactly, so far as one can determine with high powers, are clearly 
very different physiologically. In drop cultures, the first visible 
indication of a center of aggregation is seen when a few myxamoe¬ 
bae become piled up beside one another. Other individuals congre¬ 
gate toward these central points until radiating streams are formed, 
each composed of masses of numberless myxamoebae and directed 
toward the central aggregation. The larger streams, which may be 
seen even with the naked eye on the agar surface of a culture tube, 
send off branches whose ramifications soon become invisible except 
when highly magnified (pi. 7, fig. 94). Sometimes all the incoming 
colonies approach from one side, and occasionally, before reaching 
the central mass, all unite into a single stream which may creep 
some distance along the surface before rising to form a fructification 
(pi. 6, fig. 81-82). Frequently, colonies are formed simultaneously 
side by side, thus resulting in a number of gregarious fructifications 
from one base ; more often, however, they are not less than SO/a 
apart. 
The myxamoebae, when crawling toward a colony, show usually 
but few pseudopodia, and become much stretched out in the direction 
of their movement. The active individuals of Polysphondylium and 
Dictyostelium, for example, are especially elongated (pi. 6, fig. 84), 
while those of Guttulinopsis do not show this phenomenon to any 
striking degree (pi. 5, fig. 19). 
When a number of myxamoebae are united in a stream, even 
though the lines of separation between the members of the aggrega¬ 
tion may’be almost or even wholly invisible, their contractile vacuoles 
remain distinct long after they have seemingly become fused in the 
mass (pi. 6, fig. 81-82). At any stage in the development of the 
pseudoplasmodium, however, the colony may be readily separated 
into its component cells simply by crushing it in a drop of water. 
Van Tieghem (’80) is authority for the interesting statement that 
a young mass of fructifying myxamoebae of Dictyostelium, when 
transferred to a nutrient surface, will separate and the component 
individuals again pass through a stage of vegetative reproduction. 
So far as I can gather from his paper, this deduction must have been 
based on the fact that several fructifications may be produced from 
