HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 217 
My material corresponds more nearly with Nitardy's var. 
latum and Nitardy gives no evidence that his var. angustum 
ever produces dense colonies with small irregular intercellular 
spaces like the forms of P. simplex I have figured. I shall refer 
to these bilaterally symmetrical colonies and their variants as Р, 
triangulum (Ehrenb.) A. Br. (Nitardy, "14, pl. 4. f. 4, 7, 8; Ы. 5. 
f. I, 2; pl. 6, etc.) and to the irregular forms I have figured as P. 
simplex (De Wildeman, ’93, pl. то. f. 14, and perhaps Nitardy, 714, 
pl. 6. f. 3), leaving unsettled the question whether the one type 
can arise from the other directly as De Wildeman and others have 
supposed. Nitardy says he has not seen the forms without inter- 
cellular spaces such as De Wildeman has figured and evidently 
about Berlin the symmetrical form and its variants occur pre- 
dominantly. It is not impossible, of course, that the irregular 
forms are merely the expression of a lack of vigor at the swarming 
period. 
The regularity of the peripheral series in P. simplex even when 
the central cells are asymmetrically placed is doubtless due to 
the fact that the outer series of cells seems to come to rest sooner 
than those in the interior of the swarming group, as I have noted 
elsewhere (18). Тһе colonies of P. simplex with the irregular 
central groups (FIGs. 2, 3, 4) are particularly interesting as illus- 
trating a case in which while symmetrical relations of contact 
and pressure are apparently impossible for all the members of the 
cell colony they are none the less quite perfectly achieved for a 
portion of the colony—the peripheral series—so far as their inter- 
relations are concerned. 
The eight-celled colonies of P. simplex offer fewer possibilities 
in the complexity of their intercellular relations and there is also 
much greater uniformity of type. By far the commonest arrange- 
ment is one cell in the center surrounded by seven cells (FIGS. 5 
and 6). It is evident here that the single cell can hardly fill a 
circle made of seven instead of six cells (see FIGS. 5 and 9) and 
intercellular spaces tend to appear and may sometimes be quite 
large. In FIGURE 6, however, the central cell seems quite to fill 
the center of the group of eight. I have never seen a colony of 
this species with two cells symmetrically placed in the center 
and surrounded by six cells, as is so commonly the case in the 
