THE MOVEMENTS AND REACTIONS OF AMOiBA. I43 



parallel to the advancing edge (Fig. 38). These wrinkles and the 

 areas which they enclose do not change markedly as the Amoeba 

 advances, so that the outer surface of the body seems to be quite at 

 rest. It is this fact, I believe, that has prevented the true nature of 

 the movement in these species from being recognized before. Thus 

 Penard (1902, p. iiS), after a thorough study, accurate so far as it 

 goes, of the movements of Amoeba verrucosa^ notes that many facts 

 point to the existence of a permanent contractile outer layer, but holds 

 that the permanence of certain lines and patterns on the upper surface 

 in a moving Amoeba is crucial against the idea of a rolling movement 

 such as I have shown above to actually occur. In reality these 

 wrinkles are not static structures, but dynamic, /. ^., the substance of 

 which they are formed is in continual motion ; they are like the per- 

 manent ripples on the surface of a stream where the latter crosses an 

 obstruction. The wrinkles indicate the direction of movement of the 

 substance, the longitudinal wrinkles being parallel to the lines of motion, 

 the others transverse to them. A particle on the upper surface may 

 move parallel with the longitudinal wrinkles, at a constant distance 

 from them, or it may move directly along one of these wrinkles, for 

 the whole length of the latter. On coming to one of the transverse 

 wrinkles the particle moves over it with a sort of jerk, as if it had 

 passed over a ridge or step, as indeed it has. Thus the lines and the 

 areas enclosed by them remain constant, while the substance of which 

 they are composed moves onward. 



When the Amoeba changes in a marked degree its direction of 

 movement, so as to follow, for example, a course at right angles to the 

 previous one, the wrinkles on the surface usually slowly disappear, then 

 after the movement has become well established in the new direction, 

 new wrinkles appear in correspondence with the movement. 



When such a change of course occurs, any particles on the upper 

 surface, which were moving toward the anterior edge, change their 

 course in correspondence with the new direction of progression. Fig. 

 43 represents a case of this kind, where an Amoeba verrucosa bore on 

 its upper surface a minute particle of debris {a) and a spherical cyst of 

 Euglena {b). Both moved forward over the stretch x-y (Fig. 42, A^. 

 Now a little methyl green (/«) was allowed to diffuse against the left 

 side of the Amoeba. The animal changed its course, moving to the 

 right. At the same time the two objects a and b changed their direction 

 of movement, traversing the stretch j^-z (Fig. 42, B) until they reached 

 the new anterior edge of the Amoeba, and were carried underneath. 



The free-moving (upper) surface and the resting (lower) one in 

 contact with the substratum may exchange roles at any time when 

 the contact with the substratum is changed. Thus, a specimen was 



