Mat 4, 1906.] 



SCIENCE. 



709 



by the friction produced by adhesion to the 

 underlying surface, which acts in opposition 

 to the forward pressure of the endoplasm. 



This theory is based mainly upon the 

 analogous behavior of drops of chloroform 

 which creep or roll spontaneously over a thin 

 coating of shellac, with which the bottom of 

 a glass dish is covered. After the shellac is 

 dry, water is placed in the dish, and drops of 

 chloroform are introduced with a pipette. A 

 drop, under such conditions, rolls spontane- 

 ously in the opposite direction from the side 

 which first shrinks back from the surrounding 

 shellac. The adhesion between one side of the 

 drop of chloroform and the shellac results in 

 dissolving away the latter, in the diminution 

 of surface tension over this side of the drop, 

 and a rolling motion in this direction. Thus 

 the drop plows a path through the shellac, 

 propelled onward by the diminution of sur- 

 face tension at its forward margin, which in 

 the drop is produced by adhesion to the shellac. 

 The drop of chloroform soon contains shellac 

 in solution which, under the influence of the 

 surrounding water, stiffens into a ' gel,' the 

 contraction of which is supposed gradually to 

 replace surface tension, as the active agency 

 which propels the drop. If the gelatinous 

 coating is of unequal thickness and strength 

 at different points, motion occurs in the direc- 

 tion of a thin, weak point, where the contents 

 burst through the superficial envelope. 



By this analogy Ehumbler finds support for 

 the theory, long since put forth by Berthold, 

 that amceboid motion, 'at least in the amoebse 

 without backward currents, depends upon a 

 one-sided adhesion to the substratum, and also 

 for his own idea that the rolling motion of 

 such amoebse is due to the centripetal pressure 

 of the gelatinizing ectoplasm and the bursting 

 forth of the endoplasm at the anterior end, 

 which continues the adhesion to the sub- 

 stratum and forms new ectoplasm. The 

 analogy illustrates the possibility also that a 

 gradual transition may occur ontogenetically 

 and phylogenetically between the method of 

 locomotion in amcebse with fountain currents 

 and active interchange between endoplasm and 

 ectoplasm, and that in others, like A. verru- 



cosa, with ' gelatinous ' ectoplasm, which move 

 without backward currents. Such a transition 

 would depend upon the degree of gelatiniza- 

 tion that had been reached. This process 

 might be supposed to take place in A. hlattce, 

 for example, until the backward peripheral 

 flow should cease entirely, and the gelatin- 

 izing ectoplasm should reverse the direction 

 of its movement over its free surface, and 

 turn forward. 



It is evident that Rhumbler's theory rests 

 largely on an analogy with the behavior of 

 drops of non-living fluid under special condi- 

 tions. Facts which indicate that a constant 

 gelatinization and the opposite are actually 

 taking place in amoebae with rolling motions 

 are extremely meager. It seems highly prob- 

 able that A. Mattw moves in response to un- 

 equal surface tension; but Rhumbler's ex- 

 planation of the more complicated (rolling) 

 motions of other amoebse, though a useful hy- 

 pothesis, needs the support of more facts 

 drawn from direct observation of the organism 

 itself, before it can be proved. 



Jennings describes the rolling movement as 

 consisting of the outflow of a wave of pro- 

 toplasm at the anterior end of the Ammhaj 

 which exerts a pull upon the upper surface, 

 drawing it forward, while the hinder portion 

 of the Amceba becomes released from the sub- 

 stratum and contracts. Thus the ' strong pull 

 from in front and the slight contraction from 

 behind ' cause the posterior end, and the inner 

 contents, to flow forward. Jennings, accord- 

 ingly, places emphasis upon the pull from in 

 front, and looks upon it as a more important 

 factor in locomotion than the contraction of 

 the ectoplasm at the rear. 



Khumbler's theory at this point departs 

 from Jennings's interpretation of the facts. 

 The supposed gelatinizing, or semigelatinous, 

 covering of AmcBlia, according to Rhumbler, is 

 constantly bursting at its anterior edge, which 

 is attached to the substratum. Out of the 

 breach the endoplasm flows forward, as the 

 result of the contraction of the gelatinous 

 covering of the upper and posterior surfaces. 

 If these are the facts, there can be no strong 

 superficial pull exerted from in front back- 



