HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 225 
the colony as a whole. The result is a light and open structure 
which may be better suited to conditions of life in the plankton. 
The intercellular angles are hard to measure because of the 
very limited areas of contact between the cells but there can be 
no question that the variations from 120° are so slight as not to be 
accurately determinable by the means I have used. The slender- 
ness of the lobes makes possible in the highest degree compensa- 
tory curvings and bendings so as to give quite equal pressure and 
tension relations between the surfaces of contact of the cells. 
The extreme length of the lobes has brought with it a new type of 
cell grouping in the colony. I have had an abundance of material 
of this form and have not seen a single sixteen-celled individual 
with the common cell arrangement 1 4- 5 4- 10, found in P. Boryanum 
and P. asperum. 
Nitardy (pl. 8. f. 13) refers to P. clathratum, a sixteen-celled 
colony with the ordinary arrangement found in P. asperum 
1+5 + 10. The cell form also is plainly that of P. asperum 
rather than that of P. clathratum. Chodat’s (от) figures of what 
he identifies as P. duplex are both of the clathratum type. His 
figure 151 is of an irregular older colony nearer the stage of repro- 
duction, but figure 1520 shows the type configuration of the cells. 
The type arrangement seems to be that shown in the diagram 
(FIG. 16), five cells surrounded by eleven cells and the center of 
the colony an open pentagonal or oblong area. Such a colony is 
bilaterally symmetrical about the axis mz, as shown in the 
diagram. The outer series of cell contacts is in threes except at : 
the pole m, where there is a contact between two. The inner 
series of contacts is all in twos. The central intercellular space 
is, as noted, pentagonal and more or less elongated in the axis of 
the colony. With the variation in the shape of the central inter- 
cellular space the whole colony becomes either rounder or more 
oblong. Compare FIGURES 15 and 17. In correlation with the 
length and slenderness of its cell lobes P. clathratum is a climax form 
in its development of intercellular spaces. The outer series of 
intercellular spaces consists of five inequilateral lens-shaped and 
five shield-shaped openings bounded by two and three cells 
respectively, with the large oval and four-cornered intercellular 
space near the pole m, bounded by four cells and bisected by the 
