364 THE FORMS OF CELLS [ch. 



behaves very much Hke a fluid drop. So, to a first approximation, 

 even the yeast-cell shews, by its ovoid and non-spherical form, that 

 it has acquired its shape under some influence other than the uniform 

 and symmetrical surface-tension which makes a soap-bubble into a 

 sphere. This oval or any other asymmetrical form, once acquired, 

 may be retained by virtue of the solidification and consequent 

 rigidity of the membrane-like wall of the cell; and, unless rigidity 

 ensue, it is* plain that such a conformation as that of the yeast-cell 

 with its attached bud could not be long retained as a figure of even 

 partial equilibrium. But as a matter of fact, the cell in this case 

 is not in equilibrium at all ; it is in process of budding, and is slowly 

 altering its shape by rounding off its bud. In like manner the 

 developing egg, through all its successive phases of form, is never 

 in complete equilibrium: but is constantly responding to slowly 

 changing conditions, by phases of partial, transitory, unstable and 

 conditional equihbrium. 



There are innumerable solitary plant-cells, and unicellular 

 organisms in general, which, hke the yeast-cell, do not correspond 

 to any of the simple forms which may be generated under the 

 influence of simple and homogeneous surface-tension ; and in many 

 cases these forms, which we should expect to be unstable and 

 transitory, have become fixed and stable by reason of some com- 

 paratively sudden solidification of the envelope. This is the case, 

 for instance, in the more comphcated forms of diatoms or of desmids, 

 where we are dealing, in a less striking but even more curious way 

 than in the budding yeast-cell, not with one simple act of formation, 

 but with a complicated result of successive stages of localised growth, 

 interrupted by phases of partial consolidation. The original cell 

 has acquired a certain form, and then, under altering conditions 

 and new distributions of energy, has thickened here or weakened 

 there, and has grown out, or tended (as it were) tc branch, at par- 

 ticular points. We can often trace in each particular stage of 

 growth, or at each particular temporary growing point, the laws of 

 surface tension manifesting themselves in what is for the time being 

 a fluid surface ; nay more, even in the adult and completed structure 

 we have little difficulty in tracing and recognising (for instance in 

 the outline of such a desmid as Euastrum) the rounded lobes which 

 have successively grown or flowed out from the original rounded and 



