Some Physical Properties of Protoplasm 



259 



of activity, each with its own rhythm, then movement from one 

 part to another can be accounted for. 



For these reasons, I regard protoplasmic movement in myxomy- 

 cetes as not identical to that in Amoeba. There is apparently in 

 Amoeba one cortical contractile layer and one major line of flow, 

 whereas, a myxomycete consists of many regions each exhibiting 

 rhythmic contractility which is not necessarily in perfect synchron- 

 ism with its neighbors. 



The most convincing evidence against a contractile cortical layer 

 in slime mold is the wrinkled surface of a plasmodium at systole. Con- 

 traction should shorten and 

 therefore smooth out the sur- 

 face of a pseudopod as it is 

 emptied of protoplasm. The 

 lowest of the three photo- 

 graphs in Figure 3 is the 

 contracted state of the pseu- 

 dopod, yet its surface is much 

 wrinkled, whereas, the sur- 

 face of the expanded proto- 

 plasmic inass, at the top in 

 Figure 3, is smooth. Tension 

 at a surface should tend to 



round up, not buckle, the contour of a contracting fluid drop. Wrink- 

 ling of the surface with a decrease in volume because of loss of proto- 

 plasm indicates that there has been no change in the total surface 

 area and therefore, no contractility of the surface layer. This is a 

 matter of great importance for the whole concept of rhythmic con- 

 tractility in protoplasm, and is to be explained, I think, by the con- 

 clusion expressed in the foregoing paragraph, namely, that in slime 

 molds there is not one single contractile mechanism which resides in 

 the surface layer, but many independent centers of rhythmic con- 

 tractility. 



The conclusion to which I come is similar to that reached by 

 Pfeffer, and by Ewart, in relation to another physical force, that of 

 surface tension, then held to be responsible for protoplasmic flow. 

 Their deduction is as applicable to the present case as it was to their 

 own, namely, that the energy responsible for protoplasmic flow is 

 liberated throughout the living substance and not solely or primarily 

 at the boundary. 



Another important matter which should be emphasized is that 



Fig. 5. Simultaneous flow both toward 

 and away from the periphery of a Plasmo- 

 dium. 



