THE MOVEMENTS AND REACTIONS OF AMCEBA. 157 



decrease in surface cannot keep pace with it, so that the surface is 

 thrown into folds. This phenomenon is particularly interesting in its 

 bearing on the theory that would account for the retraction of pseudo- 

 podia by the action of surface tension. On this theory we should 

 naturally expect the surface to remain smooth, and by no means to be 

 thrown into folds, since it is by the tendency of the surface to decrease 

 that the decrease in volume is accounted for ; the decrease in volume 

 should not, therefore, precede the decrease in surface. This matter will 

 be taken up later. 



As the pseudopodium decreases in size, the fluid endosarc, of course, 

 flows out of it and joins the endosarc of the body. The backward 

 current begins at the mouth or inner end of the pseudopodium, and 

 gradually extends backward to near the tip ; the current is most rapid 

 in the central axis of the pseudopodium, and in this axis it is most 

 rapid at the inner end.* 



Where is the impelling force in the outflow of the endosarc and the 

 decrease in size of the pseudopodium ? The observations seem to sug- 

 gest several factors here. The fact that the ectosarc of the pseudo- 

 podium passes on to the body when the pseudopodium shortens, as is 

 shown in Fig. 49, a, b, c, indicates that the ectosarc of the body exercises 

 a pull on the outer layer of the pseudopodium, drawing it inward. 

 This would, of course, force the fluid endosarc into the body. But this 

 would not account for the wrinkling and roughening of the outer surface 

 of the pseudopodium, which is so prominent a feature in the withdrawal. 

 For this there are two conceivable causes, (i) The ectosarc itself may 

 contract actively, driving out the endosarc. If the real contractile por- 

 tion of the ectosarc is not on the outer surface (in the cuticula, as it has 

 sometimes been called), but in a deeper layer, then the outer surface 

 would be thrown into folds or prominences as contraction occurs. (2) On 

 the other hand, it is conceivable that the endorsarc might be drawn out 

 of the pseudopodium, the latter collapsing and becoming wrinkled as 

 a result. This is the explanation given by Biitschli (1892, p. 201). 

 This view would have to assume some force pulling on the endosarc at 

 the mouth of the pseudopodium, and sufficient viscosity in the endosarc 

 so that a pull thus exercised would draw out the whole mass contained 

 within the pseudopodium. Thus, in Fig. 49, , the general advancing 

 current within the body of the Amreba might be thought to exercise a 

 pull at the point y in the direction of the arrow ; if the endosarc were 



*This account differs from that given by Biitschli (1880, p. 116), according to 

 whom the withdrawal of the pseudopodium begins at the tip. The observations 

 present no difficulty, and I am unable to understand how Biitschli came to this 

 result. In a large pseudopodium the method of retraction described above is 

 evident. 



