68 



THE CELL AND PROTOPLASM 



activities, and plays a definite role in dif- 

 ferentiation. 



Nucleoplasm, whether in the form of 

 more or less scattered granules as in some 

 primitive cells, or as a definite nucleus sur- 

 rounded by a membrane, apparently origi- 

 nated under conditions existing in the cell 

 interior. At present it appears that nuclear 

 material is synthesized only in the interior 

 of a cytoplasmic mass. The nucleus, then, 

 appears to be more or less isolated from 

 external environment except through the 

 mediation of cytoplasm. 



Even though we may not agree fully 

 with the view advanced by Just in his 

 recent book on the biology of the cell sur- 

 face that the ectoplasm is the all-important 

 part of the cell, it is evident that the cell- 

 surface possesses properties important for 

 the life of the cell. The plasma membrane 

 often appears to be more or less elastic, 

 either in consequence of surface tension or 

 structurally, and so may be a factor in de- 

 termining form of the cell. It carries an 

 electric charge and a double electric layer 

 — a membrane polarization, results from 

 accumulation of ions of opposite sign on or 

 near its external surface separating it from 

 a fluid medium. 



It is more or less semipermeable ; that is, 

 it permits passage of certain substances or 

 ions exclusively or more readily than 

 others. The permeability for particular 

 substances differs in different cells. In 

 spite of the enormous amount of work on 

 permeability, we do not know in most cases 

 exactly how particular substances penetrate 

 into living cells, nor do we know to what 

 extent results obtained with certain kinds 

 of cells are of general significance. Vari- 

 ous hypotheses have been advanced : pres- 

 ence of a lipid layer and penetration of 

 substances dependent on their partition 

 coefficient between it and an aqueous 

 medium ; the membrane a mosaic of lipid 

 and water-containing regions, permitting 

 penetration of fat-soluble substances in the 

 lipid regions, water-soluble substances in 

 other regions; penetration by chemical re- 

 action with constituents of the membrane; 

 entrance through pores; penetration de- 



termined by electric charge of membrane 

 and of ions concerned, and penetration by 

 adsorption on membrane particles. 



The semipermeability of the cell surface 

 mediates and controls the material ex- 

 change between the external world and the 

 cell interior, but it is by no means inde- 

 pendent of external environment. It is 

 altered by change in temperature, by radi- 

 ation, by the electric current, by the chemi- 

 cal constitution of the medium, and ap- 

 parently often by mechanical alteration 

 such as stretching. Moreover, it does not 

 determine all relations between cell and 

 environment. Change in temperature may 

 alter permeability, but its effect on rate of 

 metabolic reactions is not dependent on 

 permeability. Action of the penetrating 

 radiations on the cell interior is indepen- 

 dent of the permeability of the cell surface. 

 Permeability is not the essential factor in 

 the effects of lack of oxygen or of certain 

 ions. 



The surface-interior pattern is not the 

 only pattern of differentiation appearing 

 in the cell. The single cell may become an 

 organism or a part of a multicellular or- 

 ganism with patterns of development re- 

 ferable to an axis or axes superimposed on 

 its surface-interior pattern. These patterns 

 are variously designated as polarity, radial 

 or bilateral symmetry, ventrodorsality, 

 dorsiventrality, etc. The question as to 

 how they originate is of fundamental im- 

 portance to our conception of living or- 

 ganisms. Do they persist from one cell 

 generation to another, having originated 

 once for all at some past time? Do they 

 arise autonomously in the cell or cell mass 

 through action of the genes or otherwise? 

 Or are they fundamentally behavior pat- 

 terns; that is, reactions to conditions ex- 

 ternal to the protoplasm concerned? That 

 at least certain of them can be established, 

 altered, or obliterated by external environ- 

 mental factors is evident from various lines 

 of experiment. A few examples will serve 

 to show something of these relations to 

 environment. 



Living protoplasms are irritable; that 

 is, they react or respond by certain changes 



