Polarity 117 



It is important at the beginning to understand exactly what is meant 

 by the term polarity. Sometimes this is regarded as an innate quality of 

 an organism which makes its parts line up in a given direction, like iron 

 filings in a magnetic field or opposite electrical charges at the two poles 

 of an electrophoretic system. How far such polarizing factors operate in 

 organisms we do not know. The term polarity as used most commonly, 

 and certainly in the present discussion, implies much less than this and 

 involves no assumption as to its causes. Polarity is simply the specific 

 orientation of activity in space. It refers to the fact that a given biological 

 event, such as the transfer of material through an organ or the plane in 

 which a cell divides, is not a random process but tends to be oriented in 

 a given direction. If this were not so, an organism would grow into a 

 spherical mass of cells, like tissue in a shaken culture. This differential 

 directiveness is responsible for organic form. What is the cause of it we 

 do not know, but one often invokes it, although as an expression of igno- 

 rance, in attempting to account for a morphogenetic fact. Polar behavior 

 is no more and no less mysterious than organic formativeness but is merely 

 the simplest manifestation of this, the tendency to develop a major axis 

 with lateral ones subordinate to it. 



It is essential to realize, however, that polarity is not a trait that is 

 originally and invariably present. There is good evidence that entirely 

 undifferentiated cells, such as eggs in their early stages and other very 

 simple ones, manifest no polarity at all. Within them, doubtless, there are 

 polar molecules but these are arranged at random, like iron filings that 

 are not in a magnetic field. Sooner or later a gradient is established in the 

 cell which lines up these molecules in a specific orientation. This orien- 

 tation originates in asymmetric factors in the outer environment, such 

 as gravity, light, or the influence of adjacent cells, or perhaps within the 

 cell from gene action. As a result, the various phenomena of polarity make 

 their appearance, but not until a gradient has first been set up. Once a 

 cell or a group of cells have thus become polarized, they will usually 

 proceed to develop into an axiate system which then produces an or- 

 ganic form without necessity for further environmental induction. 



The tendency toward polar orientation, which may be strong or weak or 

 reversible and is differentially susceptible to outer influences, is the funda- 

 mental fact of polarity. It must be distinguished from the various factors 

 of induction that call forth and make manifest this polar tendency. To say 

 that light induces polarity in the egg of Fucus is to describe a morpho- 

 genetic fact, but a different problem is to explain the character of the 

 cell that makes it capable of a specific polarization. An explanation of 

 polarity in physical and chemical terms is difficult but a beginning at this 

 task has already been made. In most biological discussions today, how- 

 ever, the term polarity is primarily a descriptive one. 



