SUBSTRATE EFFECTS 499 



Secondly, the expression of specific attraction seems to depend on 

 the conditions of spiraUsation in the chromosome threads. While on 

 the one hand the molecular spiral may cause association in the absence 

 of attraction by torsion pairing, on the other hand two separately 

 spiralised threads show less attraction than two unspiralised threads. 

 While the sudden lapse of attraction between homologous chromatids 

 at first anaphase must be due to a change in the substrate, the 

 absence of attraction between second prophase chromatids as 

 compared with that shown at all other prophases seems more likely 

 to be due to the spiralised threads having an attraction sufficient 

 to maintain an association established at an unspiralised stage but 





II ^ 



• • c 

 fa lb la U 5a Zh ^ 



Fig, 139. — Equilibrium positions of the chromatids determined 

 by different relative strengths of primary attraction and surface 

 repulsion, (i) Meiotic bivalent, (2) mitotic chromosomes, 

 (3) trabants, (4) acentric chromatids. 



not sufficient to establish a new association in the face of the repul- 

 sion existing between all threads. The strength of the specffic 

 attraction between two spiralised threads that are jointly coiled is 

 shown by the arrest of terminalisation, owing to change of homology, 

 that will be described later. 



Thirdly, the special properties of one chromosome (with differen- 

 tial condensation) as contrasted with the rest shows that these 

 differences of behaviour are also conditioned directly by material 

 differences within the chromosome. 



Another source of misunderstanding lies in the distinction that 

 may be drawn between the primary and residual attractions 

 inasmuch as the second merely modify an equilibriimi position 

 determined by repulsion while the first seem to result in " contact." 

 But the appearance of contact is not to be naively interpreted 

 on a macroscopic analogy. It is itself always a position of equili- 



