164 THE BEHAVIOR OF LOWER ORGANISMS. 



membrane seems especially well developed, for the hyaloplasm pushes 

 then a long way ahead and may form half the length of the Amoeba 

 before the endosarc has burst through the membrane. 



The phenomena described above are very general in creeping Amoebae, 

 both in those with usually but a single pseudopodium (as A. Umax), 

 and in those with many pseudopodia (as A. angulata). 



Of course, the general fact that there is a separation between hyalo- 

 plasm and endosarc is not explained by these observations ; thus we 

 know that they are often separated even in pseudopodia that are pro- 

 jected freely into the water. But the phenomena are much less marked 

 in such cases ; it is exactly the observed difference between them and 

 the phenomena to be seen in a creeping Amoeba that the above obser- 

 vations explain. 



In all these details it is important not to lose sight of the essential 

 point in the movement at the anterior end. This is as follows : The new 

 wave begins on the upper surface just behind the former boundary line, 

 and rolls forward, so that its former upper surface is now in contact 

 with the substratum. 



This method of movement explains a peculiar fact which one observes 

 frequently, and which I found it difficult to understand before this 

 movement was demonstrated. The advancing edge in Amoeba usually 

 does not push forward fine, loose particles lying on the substratum in 

 front of it, but overlaps them instead, so that the Amoeba creeps over 

 them. This is especially noticeable when the water contains many 

 particles of India ink or of soot. In view of the rolling movement with 

 the series of waves, each coming from behind the previous anterior 

 edge and thus descending on the substratum from above, this burying 

 of loose movable particles becomes intelligible. 



In Amoeba proteus and its relatives the advancing anterior edge does 

 not move forward in a single uniform curve, as it does in A. verru- 

 cosa, and, as a rule, in A. Umax. On the contrary, the anterior edge 

 may show the most varied form and modeling (Fig. 54) ; narrow points 

 may be sent out here ; a broad rounded lobe there ; a rectangular projec- 

 tion elsewhere. Pseudopodia may rise above the general level, pro- 

 jecting freely into the water and later coming in contact with the 

 substratum, or be withdrawn without coming thus in contact. The 

 anterior portion of the body may divide into two lobes, or may become 

 hollowed out so as to contain a cavity bounded by thin walls (see 

 later, Figs. 75, 76). These facts show clearly that the method of 

 advance of the anterior edge cannot possibly be determined by general 

 pressure from behind, such as would be produced, for example, by a 

 contraction of the posterior part of the body. Such pressure could not 

 produce the cavity shown in Fig. 75 or the thin edge bearing numerous 

 points shown in Fig. 54. 



