362 



AN INTRODUCTION TO MODERN GENETICS 



The identification of these forces with definite physical agents is still 

 largely speculative, but the time is ripe for an attempt to see how far 

 the theory of colloidal structures can account for them. In this attempt 

 it will be advisable, following Darlington, to consider separately the 

 forces concerned with the "internal mechanics" of the chromosomes, 

 that is to say, forces which act over distances of the same order as the 

 diameter of the chromonema, and those concerned with the "external 

 mechanics," which act over distances of the order of the length of the 



Fig. 146. Anaphase Movement. — A. Anaphase in a mitosis in a pollen grain of 

 Podophyllunn. One group of chromosomes is near the wall of the cell, and appears 

 to have ceased moving, while in the other the chromosomes are still lying parallel 

 as they are pushed into the centre of the cell. 6, C. Early and late anaphses in the 

 meiotic division of Stenobothrus lineatus, to show the elongation and narrowing 

 of the spindle between the separating groups of chromosomes. Note the mito- 

 chrondria lying near the surface of the spindle (after Belar). 



(From Darlington.) 



chromonema; we shall postpone till the next section a consideration of 

 the special forces which bring about the spiral coiling of chromo- 

 somes. 



For the internal mechanics, colloid theory provides two forces i^ a 

 repulsion due to the formation of a double electron layer on the surface 

 of neighbouring particles in a fluid medium; and an attraction due to 

 the so-called London- van der Waal's forces. Both these forces are 

 non-specific; that is to say they occur between any two particles and 

 not only between similar ones. This is what is required of the repulsive 

 forces between chromosomes and between centromeres and centro- 

 somes. On the other hand, the attractions appear to be specifically 



^ The discussion largely follows unpublished suggestions of J. D. Bemal. 



For the basic conceptions, see Freundlich 1932, Hamaker 1937, 1938. 



