364 AN INTRODUCTION TO MODERN GENETICS 



shape of the asters which are often found at the poles of the spindle 

 round the centrosomes is the equilibrium for a similar substance 

 arranged round a singular point or within a spherical wall. 



The magnitude of repulsive and attractive forces between bodies 

 within a hquid crystal will be expected to be different in different 

 directions, and in general greater when parallel to the direction in 

 which the particles are orientated. This seems to be the case with the 

 repulsions between the centromeres/ which move the chromosomes 

 along the axis of the spindle towards the poles and not centrifugally 

 outwards from the metaphase plate. This mechanism cannot account 

 for the whole of the anaphase separation unless we assume that the 

 repulsion between centromeres and centrosomes becomes ineffective 

 in the later stages. A more probable suggestion was made by Belarj^ 

 who showed that there is an active elongation and narrowing of the 

 part of the spindle (the "Stemmkorper") which Ues between the sepa- 

 rating groups of chromosomes in mid-anaphase, and claimed that in 

 the final movement the two groups are passively pushed apart by this 

 elongating material. 



2. Spiralization of Chromosomes^ 



Metaphase and anaphase chromosomes usually appear as fairly thick 

 solid rods. Suitable methods of fixation and staining (particularly pre- 

 treatment with acids or ammonia vapour) reveal more structures. The 

 chromosome consists of the chromonema thread which in mitosis is 

 coiled in a single tight spiral, while in meiosis this spiral may be coiled 

 again in a second spiral. The mitotic spiral is known as the minor 

 spiral and has up to about thirty turns. The spiral which is super- 

 posed on this in meiosis is the major spiral and has about five or six 

 turns; it has so far only been described in plants. 



Cytologists have not as yet by any means reached agreement as to 

 the phenomena of coiling. The debate is partly concerned with the 

 relation between the spirals in two paired chromosomes, and this is 

 also connected with the argument mentioned in Chap. 5 concerning 

 the time of splitting of the chromosome thread. Another imdecided 

 point is the frequency of changes of direction of the spiral from right- 

 handed to left-handed, and their possible relation to crossing-over. 

 This question also has a direct bearing on the interpretation of coiling 

 in terms of the internal structure of the chromosome. There are two 



^ Cf. Darlington 1936a. ^ Belar 1927, 1929, 



^ General references: Darlington 1937, Geitler 1934, Kuwada 1927. 



