1178 Wisconsin Academy of Sciences, Arts and Letters. 
2—6—6—2, 3—5—5—3, and 4—4—4—4. Of, these possible 
arrangements only two were found in my cultures, those of 3— 
5—5—3 and of 4 —4—4—4, the latter being much the more 
abundant. This latter arrangement is shown in the drawing 
of Senn (35). When the cells are so arranged that the differ¬ 
ent tiers contain respectively 3—5—5-—3 cells (Fig. 7 ), there 
is a single large central cavity. This peripheral arrangement 
of the cells about a central cavity was first noted by Pringsheim 
(34). 
In my opinion, the variation in cellular arrangements in the 
eight and sixteen-celled colonies of C oelastrum. depends upon 
variations in the manner of cleavage of the mother cell which 
formed these colonies. Pringsheim (34) held that this cleav¬ 
age is simultaneous : Senn (35) is in doubt as to the method of 
cleavage but thinks that it is probably progressive. It seems 
to me highly probable that Senn is correct. Instances of prog¬ 
ressive cleavage among Protoccocales nearly related to Coelast¬ 
rum have been cited above in connection with the cleavage of 
'Tetracoccus . The 4—4 arrangement in the eight-celled coenobe 
would be brought about by three successive divisions at right 
angles to one another. The 1—3—3—1 arrangement in the 
eight-celled coenobe may result from the fact that the tertiary 
divisions are not in the same plane, but that within each half 
of the original mother cell one of these planes is at right angles 
to, and the other parallel to the primary cleavage plane. The 
different arrangements found in the sixteen-celled coenobes may 
be accounted for by similar variations in the manner of forma¬ 
tion of the cleavage planes. 
This variation in the planes of cleavage is of more import¬ 
ance in forms which produce nonmotile autospores than in 
those which have motile spores. In Pediastrum and Hydro- 
didyon it matters little in what planes the cleavage takes place, 
since it is the movement of the swarm spore which determines 
the position of the cells in the adult colony. However, Harper 
(20) holds that the manner of cleavage in the formation of the 
swarm spores of Hydrodidyon is one of the determining factors 
