V] THE FORM OF AMOEBA 213 



energy of the system is for the time being at a minimum, the 

 cell assumes a rounded or spherical form, passing into a state 

 of "rest," and (for a reason which we shall presently see) 

 becoming at the same time "encysted." 



In a budding yeast-cell (Fig. 60), we see a more definite and 

 restricted change of surface tension. When a "bud" appears, 

 whether with or without actual growth by osmosis 

 or otherwise of the mass, it does so because at a 

 certain part of the cell-surface the surface tension 

 has more or less suddenly diminished, and the 

 area of that portion expands accordingly ; but in 

 turn the surface tension of the expanded area will 

 make itself felt, and the bud will be rounded of? p- gQ^ 

 into a more or less spherical form. 



The yeast-cell with its bud is a simple example of a principle 

 which we shall find to be very important. Our whole treatment 

 of cell-form in relation to surface-tension depends on the fact 

 (which Errera was the first to point out, or to give clear expression 

 to) that the incipient cell-wall retains with but little impairment 

 the properties of a liquid film *, and that the growing cell, in spite 

 of the membrane by which it has already begun to be surrounded, 

 behaves very much like a fluid drop. But even the ordinary 

 yeast-cell shows, by its ovoid and non-spherical form, that it has 

 acquired its shape under the influence of some force other than 

 that uniform and symmetrical surface-tension which would be 

 productive of a sphere ; and this or any other asymmetrical form, 

 once acquired, may be retained by virtue of the solidification and 

 consequent rigidity of the membranous wall of ^he cell. Unless 

 such rigidity ensue, it is plain that such a conformation as that of 

 the cell with its attached bud could not be long retained, amidst 

 the constantly varying conditions, 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 the bud. It is plain that over 

 its surface the surface-energies are unequally distributed, owing 

 to some heterogeneity of the substance; and to this matter we 

 shall afterwards return. In like manner the developing egg 



* Cf. infra, p. 306. 



