152 



THE BEPIAVrOR OF LOWER ORGANISMS. 



tractile vacuole; there one sees almost always very fine folds, forming angles 

 and varied patterns; these angles and these patterns remain for a long time 

 absolutely the same, which shows that nothing has changed place. (Penard, 

 1902, p. 118.) 



In all the specimens o{ Af/iceSa verrucosa and A. sphceronucleolus 

 in which I have studied the matter, the posterior part of the outer mem- 

 brane does follow the movement. Particles clingring to the outer surface 

 of the hinder part of the ectosarc pass upward over the wrinkled sac- 

 like posterior end and forward on the upper surface. In so doing they 

 pass directly across the wrinkles on the body sur- 

 face, as set forth on p. 143. Had Penard chanced to 

 see the movements of a particle attaclied to the outer 

 surface of the body he could not have been misled 

 by the apparent permanence of the surface wrinkles. 



FORMATION AND RETRAC riON OF PSEUDOPODIA. 



Thus far the phenomena in Amoeba protcus 

 and its relatives are essentially like those found 

 in Amoeba verrucosa. At times Amoeba proieus 

 flows forward as a single simple mass ; then its 

 locomotion may be compared directly in its chief 

 features to that of Amoeba verrucosa. But in 

 Amoeba proieus and its relatives the movement 

 is, of course, usually much complicated by the for- 

 mation of pseudopodia. In considering the way 

 in which these are formed we must deal separately 

 with two different cases, depending on whether the 

 pseudopodium when sent out is or is not in contact 

 with the substratum. 



SURFACK CURRENTS IN THE FORMATION OF PSEUDOPODIA 

 IN CONTACT WITH THE SUBSTRATUM. 



Fig. 46.* When the pseudopodium is sent out in contact 



with the substratum, the phenomena accompany- 

 ing its formation are essentially the same as those which take place at 

 the anterior end of an advancing Amoeba ; the latter may indeed be 

 considered as merely a large pseudopodium. Even when the pseudo- 



* Fig. 46. — Movement of a particle attached to the outer surface of a pseudo- 

 podium that is extending in contact with the substratum. At a the particle is 

 at the middle of the upper surface; at b it has nearly reached the tip. When 

 the pseudopodium has reached the length shown at c the particle has passed 

 over its tip. Here it remains, so that at d, when the pseudopodium has become 

 longer, the particle is still at the same level, but on the under surface of the 

 pseudopodium, some distance behind the tip. 



