190 



FINE-STRUCTURE OF PROTOPLASM 



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effect of this contraction is to increase their distance apart, but no 

 external tension is manifest. Only if the particles are joined by junc- 

 tions (Fig. 1 1 1 b) is a microscopically visible shortening possible and 

 an external force exerted. 



Cell polarity. Another important fact which has to be explained by 

 ^ I a consistent theory of plasma 



structure is the polarity of 

 cytoplasm. This property is 

 especially evident with the 

 A eggs of Ecliinodermata and 

 i| hi Amphibia. These cells show 

 definite animal and vegetative 

 poles. Sometimes the animal 

 pole is characterized by a pa- 

 pilla, but this is not universal. 

 There is as well an invisible 

 physiological polarity. Were 

 the structural elements of 

 cytoplasm independent of 

 each other as in a liquid, no 

 fixed polar arrangement within the cytoplasm would be conceivable. 

 The polarity, therefore, must be inherent in the plasma gel. As the 

 cortex of the egg has undoubtedly a gel-like character and in this 

 state is capable of considerable active transformation when the 

 fertilization membrane is formed (Runnstrom, 1944), one might be 

 inclined to attribute the polar properties to this cortical layer. But 

 MoNNE (1946 b) finds that there is a dorsoventral gradient also within 

 the egg, the animal cytoplasm being more solidified and the vegetative 

 cytoplasm more liquefied. It is admitted that the heteropolar organi- 

 zation of the egg is predetermined by the foregoing cell division 

 (Lehmann, 1945). Cytoplasmic currents do not destroy the hetero- 

 polar organization. From this fact I suppose that important junctions 

 of the protoplasmic framework are still present throughout the moving 

 cytoplasm. As Monne points out, cytolysis of the sea-urchin egg is 

 preceded by violent protoplasmic currents. This increased movement 

 is due to a complete liquefaction of the cytoplasm, which is followed 

 by the disorganization and the death of the cell. Complete disinte- 

 gration of the junctions, therefore, will never occur in living cells. 



Fig. III. Contraction of protein molecules; a) 



without being interlinked, h) when interlinked 



by junctions (from Frey-Wyssling, 1947) 



