360 THE F0RM8 OF CELLS [ch. 



the creature is always moving, from one protean configuration to 

 another; its surface-tension is never constant, but continually 

 varies from here to there. Where the surface tension ^is greater, 

 that portion of the surface will contract into spherical or spheroidal 

 forms; where it is less, the surface will correspondingly extend. 

 While generally speaking the surface-energy has a minimal value, 

 it is not necessarily constant. It may be diminished by a rise of 

 temperature; it may be altered by contact with adjacent sub- 

 stances*, by the transport of constituent materials from the interior 

 to the surface, or again by actual chemical and fermentative change ; 

 for within the cell, the surface-energies developed about its hetero- 

 geneous contents will continually vary as these contents are affected 

 by chemical metabolism. As the colloid materials are broken down 

 and as the particles in suspension are diminished in size the "free 

 surface-energy" will be increased, but the osmotic energy will be 

 diminished!. Thus arise the various fluctuations of surface-tension, 

 and the various phenomena of amoeboid form and motion, which 

 Biitschli and others have reproduced or imitated by means of the 

 fine emulsions which constitute their "artificial amoebae." 



A multitude of experiments shew how extraordinarily dehcate is 

 the adjustment of the surface-tension forces, and how sensitive they 

 are to the least change of temperature or chemical state. Thus, 



* Haycraft and Carlier pointed out long ago {Proc. R.S.E. xv, pp. 220-224, 

 1888) that the amoeboid movements of a white blood-corpuscle are only manifested 

 when the corpuscle is in contact with some solid substance: while floating freely 

 in the plasma or serum of the blood, these corpuscles are spherical, that is to say 

 they are at rest and in equilibrium. The same fact was recorded anew by 

 Ledingham (On phagocytosis from an adsorptive point of view, Journ. Hygiene, 

 XII, p. 324, 1912). On the emission of pseudopodia as brought about by changes 

 in surface tension, see also {int. al.) J. A. Ryder, Dynamics in Evolution, 1894; 

 Jensen, L'eber den Geotropismus niederer Orgahismen, Pfliiger's Archiv, liii, 1893. 

 Jensen remarks that in Orbitolites, the pseudopodia issuing through the pores of 

 the shell first float freely, then as they grow longer bend over till they touch the 

 ground, whereupon they begin to display amoeboid and streaming motions. 

 \'erworn indicates {Ally. Physiol. 189o, p. 429), and Davenport says {Exper. 

 Morphology, ii, p. 376), that "this persistent clinging to the substratum is a 

 ' thigmotropic ' reaction, and one which belongs clearly to the category of ' response '. " 

 Cf. Putter, Thigmotaxis bei Protisten, Arch. f. Physiol. 1900, Suppl. p. 247; but 

 it is not clear to my mind that to account for this simple phenomenon we need 

 invoke other factors than gravity and surface-action. 



t Cf. Pauli, Allgemeine physikalische Chemie d. Zellen u. Gewebe, in Asher-Spiro's 

 Ergebnisse der Physiologic, 1912; Przibram, Vitalitdt, 1913, p. 6. 



