156 The Structure of Protoplasm 



animate gel is accelerated by metabolic conditions established in 

 the cell, and also that these metabolic conditions create differences 

 in the contractile force in different parts of the plasmagel system. 

 In an Amoeba, for example, there would need to be a gradient such 

 that the contractile properties are greater in the older posterior 

 parts of the plasmagel than in the more recently formed anterior 

 parts of the system. In addition to the descriptive evidence for such 

 a gradient, which is provided by the work of Mast and of Lewis, the 

 pressure experiments of Marsland and Brown indicate a distinctly 

 lesser rigidity in plasmagel walls near the extremities of the pseudo- 

 podia, and a loss of this property would indicate, no doubt, a lesser 

 contractile potentiality. 



An extension of the quantitation data yielded by the recent 

 experiments of Noburo Kamiya ('40) and of Norris ('40) may, 

 perhaps, provide a basis for deciding whether or not the elastic 

 contractile forces generated in a plasmagel system are adequate to 

 account for the vigor of the streaming. Fortunately both of these 

 workers are using the same organism, one of the myxomycetes, 

 PhysaruTTi polycephalum. The ingenious method of Kamiya appears 

 to give very accurate measurements of the flow pressures which are 

 developed as the protoplasm streams back and forth in the channel 

 between two parts of the plasmodium, and it is to be hoped that 

 Norris' technique of determining the elastic properties of the gel 

 system can be adapted to measure the changes of tension which 

 must occur concomitantly with the rhythmic alternations in the 

 direction of flow. 



The viewpoint expressed by Seifriz ('37) represents somewhat 

 of a departure from the ideas of Lewis and Mast. Time-lapse motion 

 pictures enabled Seifriz to demonstrate very plainly that each 

 portion of the plasmodium of the slime mold displays alternating 

 periods of expansion and contraction and led him to believe that 

 these pulsations are analogous to the rhythmic beating of visceral 

 muscle as controlled by the autonomic nervous system. Funda- 

 mentally, perhaps, there may be little difference between the two 

 viewpoints, for in the last analysis it may be that the same forces 

 are involved in the setting and contraction of certain gels and the 

 contractile processes in the myofibrillae. If this be true, the marked 

 differences, in regard to the consequences of hydrostatic compression, 

 between muscle cells and cells which are characterized by proto- 

 plasmic streaming, are due to more superficial factors, as for 



