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. i). 



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 Eudorina 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 



