HARPER: BINARY FISSION AND SURFACE TENSION 159 
the free surface of the cell and tend to give us the oblong cell form 
shown in the figures. That elongation precedes or accompanies divi- 
sion is indicated by the number of oblong cells in the older colony 
shown in figure 7, plate II. 
If the four cells should remain together and flattened upon each 
other the successive divisions at right angles would give very variously 
shaped and far from rounded daughter cells, as shown in Biitschli’s 
diagram (’83, Fig. 1). 
In effect, as viewed from the pole, the eight cells come to lie in an 
up-and-down zigzag line instead of forming an in-and-out zigzag, 
thus enabling each cell to remain as nearly as possible isodiametric 
without reducing the compactness of the group. 
It has been generally agreed that each of the eight cells of the wheel 
figure divide to give a sixteen-celled stage and that the spherical or 
ellipsoidal form of the colony may be achieved (as it obviously is in 
sixteen-celled colonies of Eudorina) in this sixteen-celled stage. 
Goroschankin (’75) held that the 16- and even the 32-celled stage 
in Exudorina is a plate-shaped disk and that the transformation to a 
globular form came rather suddenly with the gelatinizing of the cell 
walls. He has not been followed in this view by later writers, though 
they have very generally been inclined to accept his account of the 
order of the cell divisions. Braun (’75), Biitschli (83), Overton (’89), 
Klein (’90), and others hold that the spherical or rounded form of the 
colony is practically achieved by the sixteen-celled stage. 
The fourth division is then a binary fission of all eight of the cells 
giving the sixteen-celled stage. The radial elongation of the four 
central cells on their free surfaces is followed naturally according to 
Hertwig’s law by their transverse division. (See Klein’s figure of 
Eudorina, ’90, Taf. V, Fig. 63.) The four peripheral cells of the 
eight-cell stage also elongate before dividing, just as in the four-cell 
stage and divide transversely, giving the sixteen-celled group. 
In Gonium the cells glide upon each other so as to form a series of 
groups of three with the central square opening, as I have pointed 
out elsewhere (12). The greater adhesion of the four central cells in 
Volvox and the elongation of the cells before division prevent the 
Gonium configuration. The same situation develops as in the pre- 
ceding third bipartition. There are eight new cells formed and these 
tend to more than fill the space on the margin of the curved disk of 
eight. Such a group is in very unstable equilibrium. The rounded 
shape of the mother-cell cavity influences the direction in which the 
cells glide upon each other, and adhesion tends to develop the com- 
pact groups of three. Of the eight new marginal daughter cells the 
four coming from the original group of four form a part of the equa- 
12 
