AMEBOID MOVEMENT 85 



cells, and it is not surprising therefore that explanations of 

 streaming and ameboid movement have taken a different direc- 

 tion among botanists than among zoologists. It is for this reason 

 doubtless that Ewart ('03), while espousing the surface tension 

 theory as explaining streaming, does not look to the superficial 

 surface of a plant cell as the source of the necessary energy, but 

 to the interior of the protoplasm. This idea is, of course not 

 entirely original with Ewart, for Biitschli, as we saw, believed 

 that protoplasm has an emulsoid structure; but according to 

 Butschli's hypothesis, the surface forces were not brought into 

 play in movement until the droplets of enchylema spread over 

 the surface and so reduced the tension. Ewart, however points 

 out that there is very much more surface energy present in the 

 interior of streaming protoplasm than is required for all the 

 movements known to protoplasm, including muscular contrac- 

 tion. According to Ewart's hypothesis the emulsion globules (dis- 

 perse phase) have their surface tension lowered at correspond- 

 ing points by electrical currents traversing the endoplasm, the 

 electrical currents themselves originating in chemical actions. 



While all available evidence from the study of colloidal solu- 

 tions and from observation from protoplasm confirms Ewart's 

 statement that more than sufficient energy is available in the 

 interior of colloids for all purposes of movement, there is little 

 or no evidence that the proper electrical currents are present to 

 release or transform the surface energy into that of movement. 

 This step in his explanation is therefore highly hypothetical and 

 at present unconvincing. Moreover, this step in the theory would 

 not be applicable to streaming as observed in amebas, without 

 very considerable modification. 



Recently Hyman ('17) has developed the surface tension theory 

 of movement in the direction indicated by Ewart. The motive 

 power is supposed to have its source in the contractility of the 

 ectoplasm. The endoplasm is held to be a passive stream, not an 

 active stream as Ewart supposed to be the case in plant cells. 

 The power of contractility is held to be due to the process of 

 gelation of endoplasm into ectoplasm, which is due to a change 

 of phase, the fluid part of the endoplasm becoming dispersed and 

 thereby developing surface energy in proportion as the amount 



