VIII] OF SIGMOID PARTITIONS 577 



tends to appear in a small, rather than a large cross-section of the 

 cell: but we are also ascribing to the cell a longitudinal polarity 

 (Fig. 227, A), and imphcitly asserting that it tends to divide (just 

 as the segmenting egg does), by a partition transverse to its polar 

 axis. Such a polarity may conceivably be due to a chemical 

 asymmetry, or anisotropy, such as we have learned of (from 

 Macallum's experiments) in our chapter on Adsorption. Now if the 

 chemical concentration, on which this anisotropy or polarity (by 

 hypothesis) depends, be unsymmetrical, one of its poles being as it 

 were deflected to one side where a little branch or bud is being 

 (or about to be) given off — all in precise accordance with the 

 adsorption phenomena described on p. 4607-then oiir "polar axis" 

 would necessaiily be a curved axis, and the partition, being con- 

 strained (again ex hypothesi) to arise transversely to the polar axis, 

 would lie obHquely to the apparent axis of the cell (as in B or C). 

 And if the oblique partition be so situated that it has to meet the 

 opposite walls (as in C), then, in order to do so symmetrically (i.e. 

 either perpendicularly, as when the cell-wall is already solidified, or 

 at least at equal angles on either side), it is evident that the partition, 

 in its course from one side of the cell to the other, must necessarily 

 assume a more or less S-shaped curvature (D). 



The complex curvature of the partition-walls in such cases as 

 these m^y be illustrated by the following experiment. Set two 



plates of glass (as in Fig. 228) in a wire . 



frame, so that they may he parallel or / / 



at any angle to one another; and dip ^ — ■ — 

 the whole thing in soap-solution, so that 



a sheet of film is formed between the 



two plates and is framed by the two / 



wires which carry them. The film is, ^ 



of course, a surface of minimal area ; its ^^' 



mean curva,ture is constant everywhere, and (since the film is an 

 open surface with identical pressure on both sides) the mean curvature 

 is everywhere nil. A related condition is that the film must meet 

 its solid framework, glass or wire, everywhere at right angles or 

 *' orthogonally"; and this last constraint leads to curvatures of 

 extreme complexity, which continually vary as we rotate one plate 

 on the plane of the other. 



