THE MOVEMEVTS AND REACTIONS OF AMCEBA. 



*59 



The processes occurring in a retracting pseudopodium are the same 

 as those taking place at the posterior end in a moving animal. In fact, 

 the cases are really identical ; the posterior portion of the body may be 

 considered merely a large retracting pseudopodium. Often, as we shall 

 see, it is impossible, from its method of formation, to consider it any- 

 thing else. 



The pseudopodia are, of course, usually formed in the anterior part 

 of the Amoeba, in contact with the substratum, and are directed in the 

 line of progression, or 

 at a slight angle with 

 the line of progression. 

 As they are withdrawn, 

 their surface, as we have 

 seen, usually becomes 

 folded or roughened, 

 and the small roughened 

 projection resulting 

 f r o m the withdrawal 

 lasts, as a rule, for a long 

 time. As the Amoeba 

 continues its forward 

 course, the base of the 

 retracting pseudopo- 

 dium retains nearly its 

 original position, as do 

 the other parts of that 

 layer of the ectosarc that 

 is against the substra- 

 tum, so that the body 

 of the Amoeba gradually 

 passes the pseudopo- 

 dium, and the latter 

 finally becomes united with the posterior end. During this transference to 

 the rear, the pseudopodium usually changes its position (Fig. 51). At 

 first it is directed nearly forward (#), then it takes a position at right 

 angles to the body (), and finally swings around with its point directed 



FIG. 51.* 



* FIG. 51. Successive stages in the retraction of a pseudopodium. At a the 

 pseudopodium extends forward at the anterior edge ; at b it has partly withdrawn 

 and stands at right angles to the body, which has partly passed it; at c the 

 pseudopodium is directed backward, and is in the posterior part of the body; at 

 d the small roughened remnant of the pseudopodium has nearly united with the 

 tail. At x the successive positions occupied by the withdrawing pseudopodium 

 are shown in a single diagram. 



