HARPER: BINARY FISSION AND SURFACE TENSION 157 
up of the eight cells by surface tension tends to conform to the shape 
of the cavity of the mother cell. The whole is an expression of the 
incompatibility of the principles of surface tension and binary fission 
complicated still further by the rather firm adhesion of the cells to 
each other. If division had produced seven instead of eight cells 
and if they were free to adjust their interrelations in accord with their 
capacity to achieve a position in which their pressure relations were 
as nearly as possible mutually compensatory, we might have the 
typical least surface group of one surrounded by six in one plane. 
If ten cells were produced by division and if, as in Hydrodictyon, the 
rounded form of the mother cell were a dominating factor we might 
get one cell surrounded by five in the form of a saucer conforming to 
the curved surface of the mother cell. A further series of five added on 
the margin of the saucer and the figure could be closed by the re- 
maining cell. If the eight cells produced by binary fission were free 
as in Pediastrum, we might get a group like the typical eight-celled 
colony of P. Boryanum with two inversely bilaterally symmetrically 
placed central cells and two groups of three peripheral cells also 
inversely bilaterally symmetrically placed with reference to each 
other (’16). Pressure of the mother cell might make the group slightly 
concave. 
With eight cells produced by bipartition from four strongly ad- 
herent mother cells and themselves rather firmly adherent the familiar 
concave cross figure is the best approximation to a least surface 
configuration. 
I have noted that in Volvox there is growth of the daughter cells 
intercalated between the divisions. This is very slight at first. In 
the early stages, as has been generally noted, the mass of the young 
colony seems little larger than that of the mother cell. In the preparda- 
tion for the third division, however, there is a marked elongation of 
the four cells. 
In the two-celled stage the halves appear symmetrical, or one cell 
may be slightly oblique (Fig. 1, Pl. II). In the four-celled stage the 
sectors at first appear quite symmetrical and uniform in appearance 
(Figs. 2, 3, Pl. II) but with the preparation for the third division a char- 
acteristic change in the form of the cells is observed. This growth 
period intercalated between the cell divisions is an essentially differ- 
entiating metaphytic character and makes possible in Volvox as in 
higher types the formation of the indeterminately large and many- 
celled colony as contrasted with the fewer-celled colonies of Goniwm, 
Pediastrum, etc., in which the cell-division stages are sharply separated 
from the cell-growth stages. The growth in the four-celled stage of 
Volvox is quite specific in that it is not a mere swelling of the cell in 
