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 Pediastriim, 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 ('i6). 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 prepara- 

 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. i, PI. II). In the four-celled stage the 

 sectors at first appear quite symmetrical and uniform in appearance 

 (Figs. 2, 3, PI. 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 Goniiim, 

 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 



