HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 219 
light on the question as to the relative abundance of individuals 
of the highest vigor as compared with those of high vigor but not 
the maximum. 
It is evident that when in swarming the ring form is by accident 
once achieved it tends to persist, since it gives, as shown by its 
contours, the most perfectly symmetrical interrelations possible 
for the eight simplex cells. Just why the cells should tend to 
find their final resting position in a situation of equal or balanced 
pressures and contacts instead of unequal or unbalanced pressures 
and contacts is the same question here as in the case of other 
coenobes. There are obviously two factors or sets of factors 
involved in all these adjustments. First, it is plain that during 
the slow and protracted writhings of the swarm-spores of vigorous 
colonies the direct physical tendency of such viscid, semi-fluid 
droplets to adhere and yet as far as possible round up and assume 
a least surface configuration will have the fullest possible oppor- 
tunity to come to expression. In the random movements of the 
swarm-spores these constantly acting physical relations will tend 
to maintain any accidentally achieved position which is comform- 
able with them and to act asa check оп any movement unconform- 
able with them. Results of this sort, however, will be chiefly 
in evidence in the later stages of colony formation. The general 
arrangement of the swarm-spores in a plate or ring must be regarded 
as the result of the interrelations of the cells as free motile organ- 
isms involving polarities, tropisms, etc., such as are observed in 
other morphogenetic processes. 
The grouping of eight cells in a ring rather than in a plate of 
seven about one in the case of P. simplex, which we are consider- 
ing, brings out most clearly the relations of two divergent types 
of activity. The unconformability of a group of seven units 
about one—instead of six about one—with the principle of least 
surfaces as applied to the whole group is what prevents any 
swarm-spore that accidentally comes into the center of such a 
group (FIG. 6) from achieving equal contact and pressure relations 
with all the cells about it and thus leads to its changing its posi- 
tion until in the present case it makes one of a ring of eight (FIG. 7). 
This is a matter of intercellular reactions involving contact and 
pressure, polarities, tropisms, etc., the physical unconformability 
