4 82 DE VELOPMENT 



obvious that, within certain limits, a longer path through a more permeable 

 mass would be more advantageous than a shorter path through a less perme- 

 able 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 hyper- 

 tonic solution of common salt is placed on either side of this central drop. 

 In a short time the pigment seems drawn out into threads {fiLros) 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 

 bismuth 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 cell division consists in the division of the 

 centrosonie into two equal parts which draw away from one 

 another. A field of force is set np 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 mentioned above. On the 

 " outer " side of the centrosomes can be seen starlike radiations 

 (astral rays) recognisable as indications of incomplete lines of force 

 which run externally to those stronger interpolar lines which 

 constitute the achromatic spindle. 



The chromatin of the nucleus is drawn into a continuous thread, 

 making a skein, which is then broken into short V-shaped lengths, 

 the chromosomes, of which there are 24 in human somatic cells. 

 These chromosomes become arranged round the equator of the 

 achromatic spindle with the apex of the V pointing to the centre. 

 This completes the first or prophase. 



The second phase of karyokinesis conmiences by the longitudinal 

 splitting of each chromosome (metaphase). One longitudinal 

 half of each chromosome then passes (anaphase) to each pole of the 



