HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 231 
tion which is so common in the Diactinia. In the eight-celled 
colonies the common arrangement is 1 + 7, with the central 
cell having the appearance of being rather crowded and suppressed 
in its development though quite regularly showing the narrow 
notch characteristic of the cell form of the species (FIG. 24). 
This apparent crowding of the central cell in a group of I +7 
is quite contrary to what one observes in P. simplex and other 
species. It is due to the pronouncedly wedge-shaped form of 
the cells. That the cell form is, as in other species, hereditary 
and not dependent on the pressure of adjacent cells for its develop- 
ment is shown in FIGURE 26, which represents an irregular four- 
celled colony, one of whose cells is almost free and has none the 
less developed the wedge-shaped form. 
he quadrifid cell form apparently does not lend itself to the 
formation of symmetrical least surface configurations with regular 
intercellular spaces as does the duplex form. I have seen no 
colonies in which there was any indication of the utilization of the 
quadrifid character in the interior cells of a colony in developing 
symmetrical intercellular relations. Braun’s figures of P. Rotula 
(55, pl. 6. f. 5-12) suggest that such cases may exist. Braun's 
Fic. 27, a, b, c, d. Reproduced from Nitardy (14). a, b, c, P. iriangulum 
(Ehrenb.) A. Br.; d, P. Rotula Ehrenb., sixteen-celled colony showing bilateral 
symmetry. 
figure (’55, pl. 6. f. 5) and Cooke's figure (984, pl. 18. f. 2d) are 
fine examples of bilateral symmetry in eight-celled colonies of 
P. Rotula Ehrenb. In the sixteen-celled colonies the central 
group of four ог five may make a ring with a four- or five-sided 
intercellular space in the center and the peripheral cells may 
also make a very perfect ring-formed series of eleven or twelve. 
There are, however, no relations of symmetry further than this 
general concentricity between the cells of the two series shown 
