I CYTOPLASM 187 



In cells with amoeboid movement, protoplasmic flow is maintained 

 by continuous gel-sol transitions. The hind part of the cell contracts, 

 and simultaneously part of the gel-like ectoplasm is converted into 

 liquid endoplasm. This can be observed directly, because particles 

 enclosed in the ectoplasm become mobile, show increased Brownian 

 movement and finally are carried away by the endoplasm. In the front 

 part of the amoeba, inner pressure causes the skin to become thin 

 and bulge outward as a pseudopodium. The invading stream of 

 endoplasm solidifies into a gel at the side walls of the bulge and 

 thus rebuilds the skin at the same rate at which the amoeba moves 

 forward. To explain protoplasmic flow we need, therefore, a deeper 

 understanding both of the contraction and of the gel-sol transition of 

 protoplasm. 



In cytoplasm liquefied artificially (by high pressure, p. 172), any 

 flow there may be stops; not only does the creeping motion of 

 Amoeba cells come to an end, but also the rotation in Elodea cells. 

 The process of cell division is interrupted also in sea-urchin eggs, 

 which display incipient constriction. If the high pressure is not main- 

 tained too long, the cytoplasm re-solidifies into a gel on return to 

 normal pressure, and protoplasmic flow and cell division resume their 

 normal course again. These experiments show that the plasma sol is 

 not capable of flowing and of forming constrictions such as those 

 necessary for cell division, since no gel structure is present to provide 

 the necessary forces. 



Lewis (1942) has shown that in sol-gel transitions the solidifying 

 protoplasm can contract. In the division of fibroblasts, for instance, 

 the division of the nucleus is accompanied by the occurrence of a 

 thickened ring of plasma gel, which divides the cytoplasm into two 

 parts by contraction. This explains how the ectoplasm of the Amoeba 

 can exert pressure on the endoplasm. 



It is of particular interest that these contractions take place rhyth- 

 mically. With the aid of time lapse photography (80 fold speeding up), 

 Seifriz (1937) has shown that the flow is a pulsating movement. 

 Kamiya (1940, 1942) succeeded in analyzing the rhythmic flow of 

 the cytoplasm in ? plasmodium strand of Physarum polycephalum by 

 means of variable one-sided counter-pressures which exactly balance 

 the flow. He observed complicated oscillatory changes in pressure, 

 which can be resolved into pure sine oscillations by Fourier analysis. 



