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STRUCTURAL DIFFERENTIATION OF CYTOPLASM 



G. W. SCARTH 



McGill University 



Theories as to the basic structure of protoplasm usually assume 

 that there is a single master key to the problem. But if differ- 

 entiation exists in the physical properties of different regions 

 of the cell, it is necessary to consider the question of correspond- 

 ing differences in submicroscopic structure. The present paper is 

 concerned with the cytoplasm and, because of considerations of 

 time and relative importance, will deal only with the continuous 

 ground substance, omitting the various inclusions. 



The type of cytoplasmic differentiation which seems to be most 

 general is that of a series of concentric zones, not usually defined 

 by any sharp vi.sible boundary but each showing distinctive phys- 

 ical properties when studied experimentally. 



INTERIOR VS. CORTEX 



As regards animal cells, viscosity tests of all kinds agree in 

 proving that the bulk of the internal cytoplasm of echinoderm eggs 

 and sometimes of amoeba and other protozoa is relatively fluid. 

 The same seems to be the case with most kinds of animal cells 

 though there appear to be exceptions such as epidermal and cer- 

 tain other epithelial cells. Along with the usual fluidity of the 

 interior, its water miscibility, solvent properties, and electrical con- 

 ductivity point to its being a sol with aqueous dispersion medium. 



On the other, hand, a cortical zone of varying thickness always 

 has much higher consistency. This region combines elastic with 

 plastic or fluid qualities to a degree which is rarely if ever paralleled 

 in physical systems; the combination becomes more intelligible in the 

 light of its marked capacity for undergoing rapid reversible gel- 

 sol transformation. In amoeboid and other movements involving 

 vortical streaming, not only do reciprocal sol-gel changes occur 

 but the cytoplasm circulates between the internal and cortical 

 regions. However, the whole of the cortex does not liquify at 

 once; some coherent structure persists as a basis of organization. 



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