204 THE BEHAVIOR OF LOWER ORGANISMS. 



equal in size, or when it is the smaller piece that begins to envelop 

 the larger, the process results differently. After one piece has been 

 drawn far into the other (Fig. 77, 4), both seem to contract strongly, 

 whereupon the connecting band of ectosarc breaks, the partly inclosed 

 piece is squeezed out of the other ; and the two separate. Usually 

 each retains its form for a few seconds after separation ; one bearing 

 a slender truncate pyramid or cone, while the other shows a deep 

 depression corresponding to this pyramid (Fig. 77, 5). After a time 

 both halves change form and move away. Usually the half which 

 partly inclosed the other becomes active long before the other, but 

 this is not invariably true. 



In a large number of cases observed it was the part which contained 

 the nucleus that attempted to envelop the other half. In order to 

 determine whether the nucleus plays a necessary part in this perform- 

 ance, I tried the following experiment: After the half which had 

 no nucleus had again become active and was moving about, I cut it in 

 two, as before. Now one half of this piece partly enveloped the other 

 in the usual way, thus showing that the nucleus is not necessary for 

 this reaction. 



These results should be compared with Penard's observations on 

 injured specimens of A. terricola, noted on page 180 of the present 

 paper. 



As to the question which these experiments were originally intended 

 to answer, whether two pieces of a single Amoeba would reunite 

 after separation, my results were negative. After the two pieces had 

 begun to move about freely they were induced to come in contact, or 

 sometimes they came in contact accidentally ; but in no case was there 

 any union. Prowazek (1901, p. 93) obtained the same result with 

 small species of Amoeba, but in a larger undetermined species he suc- 

 ceeded in bringing about a union of pieces not only from the same 

 individual, but from different individuals. 



PHYSICAL THEORIES AND PHYSICAL IMITATIONS OF 

 AMOEBOID MOVEMENTS. 



THE SURFACE TENSION THEORY. 



The movements of Amoeba as presented by Biitschli (iSSo, 1892) 

 and Rhumbler (1898) (see Figs. 30-33) are exactly those of a drop of 

 fluid moving as a result of a local change in surface tension (Fig. 34) . 

 It was, therefore, natural to assume that the cause of the movements is 

 the same in the two cases. This is the view taken by the two authors 

 named. According to Biitschli, Rhumbler, and many other authors, 

 Amoeba is a drop of complex fluid which moves about as a result of 

 local changes in surface tension. 



