v] THE FORM OF AMOEBA 363 



In their amoeboid phase the various Amoebae are just so many 

 varying distributions of surface-energy, and varying amounts of 

 surface-potential*. An ordinary floating drop is a figure of equi- 

 librium under conditions of which we shall soon have something to 

 say; and if both it and the fluid in which it floats be homogeneous 

 it will be a round drop, a "figure of revolution." But the least 

 chemical heterogeneity will cause the surface-tension to vary here 

 and there, and the drop to change its form accordingly. The httle 

 swarm-spores of many algae lose their flagella as they settle down, 

 and become mere drops of protoplasm for the time being; they 

 "put out pseudopodia" — in other words their outline changes; and 

 presently this amoeboid outHne grows out into characteristic lobes 

 or lappets, a sign of more or less symmetrical heterogeneity in the 

 cell-substance. 



In a budding yeast-cell (Fig. 103 A), we see a 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 tension has diminished, and 

 the area of that portion expands accordingly ; but 

 in turn the surface-tension of the expanded or ex- 

 truded portion makes itself felt, and the bud 

 rounds itself off into a more or less spherical form. ^^' 



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

 principle. Our whole treatment of cell-form in relation to surface- 

 tension depends on the fact (which Errera was the first to give clear 

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

 impairment the properties of a liquid filmf, and that the growing 

 cell, in spite of the wall by which it has begun to be surrounded, 



the physical nature of this so-called membrane. It behaved more or less like a fluid 

 lipoid envelope, immiscible with its surroundings. It is easily injured and easily 

 repaired, and the well-being of the internal protoplasm is said to depend on the 

 maintenance of its integrity. Robert Chambers, Physical Properties of Protoplasm, 

 1926; The living cell as revealed by microdissection, Harvey Lectures, Ser. xxii, 

 1926-27; Journ. Gen. Physiol, vm, p. 369, 1926; etc. 



* See (int. al.) Mary J. Hogue, The effect of media of different densities on the 

 shape of Amoebae, Journ. Exp. Zool. xxii, pp. 565-572, 1917. Scheel had said 

 in 1889 that A. radiosa is only an early stage of ^. proteus {Festschr. z. 70. Geburtstag 

 0. V. Kuptfer). 



t Cf. infra, p. 561. 



