382 DEVELOPMENT 



attracted towards the poles and will tend to lie on the lines of stress. On 

 the other hand, bismuth filings are polarised in a sense opposite to that of 

 the adjacent field. They are forced by the incidence of stress to move (or 

 because of friction, to tend to move) from the stronger to the weaker parts 

 of the field and thus take up positions as far from the poles as possible. 

 In general, a body placed in a field offeree will tend to move towards regions 

 of greater or less intensity of stress according as its " permeability " to the 

 particular form of energy in question is greater or less than that of the surround- 

 ing medium. 



The introduction of an aggregate of high permeability into the field of 

 force makes considerable changes in its configuration. Suppose a small 

 heap of filings were placed in the magnetic field already referred to, so that 

 it lay somewhat out of the interpolar axis but on the equatorial axis, the 

 result would be to provide an " easier path " for the lines of force. It is 

 obvious that, within certain limits, a longer path through a more per- 

 meable mass would be more advantageous than a shorter path through a 

 less permeable medium, and so many of the lines of force would be " short- 

 circuited " through the heap of filings. If, moreover, the heap of filings 

 were free to move, they would be drawn en masse into the field of force until 

 a point of equilibrium was reached. This resting place would depend for 

 its position on the relative " permeability " of the filings in heap and the 

 filings distributed over the field. 



We have dealt with a magnetic field of force because it is easy to demon- 

 strate and may be readily modified experimentally, but our remarks are 

 applicable to any field of force. A simple experiment, due to Leduc, 

 shows that the lines of stress set up by diffusion may be made manifest. 

 A layer of salt solution is spread over a flat sheet of clean smooth glass, 

 and on top of this is placed a small drop of indian ink or blood. A drop 

 of a hypertonic solution of common salt is placed on either side of this 

 central drop. In a short time the pigment seems drawn out into threads 

 (pros) stretching between the centres of the two salt drops, so making 

 a figure exactly the same as that formed by iron filings under bipolar 

 magnetic influence. 



The Bjerknes phenomenon demonstrates the applicability of this treat- 

 ment to the stresses and strains set up in a fluid as the result of movement 

 in it. Bodies synchronously vibrating or pulsating in a liquid medium 

 attract or repel one another according as their oscillations are identical 

 or opposite in phase. That is, a bipolar field exists which may have, like 

 a magnetic field, similar or dissimilar poles. In such a field of force 

 currents are set up in the fluid (hydrodynamic lines of force) and any 

 particles in suspension, if lighter than the fluid, act like the iron filings, if 

 heavier like the hismuth filings above. Moreover, the lines of force set up 

 by identically pulsating (attractive) bodies are exactly similar to those 

 produced by similar (repulsive) magnetic poles, and vice versa. 



The first stage in the development of a cell consists in the 

 division of the centrosome into two equal parts which draw 

 away from one another. A field of force is set up between the 

 two centrosomes and threads of those cell constituents which are 

 more " permeable " to the form of energy existing, are carded 

 by the incident stress into a figure closely resembling those 



