io6 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



This last point of view has suggested the expression terminalisaUon 

 to describe the change (D., 1929 c). 



However many chiasmata are terminaHsed, the chromosomes 

 remain associated by terminal chiasmata. It must therefore be 

 assumed that they merge at the ends, for if they fused interstitially 

 they would sometimes cancel one another out. Why they do not 

 do so when they fuse at the ends is a problem that will be specially 

 considered later (Ch. XII). 



In many organisms, such as Tiilipa and Zea, a slight movement of 

 chiasmata takes place, but there is usually no fusion at the ends and 

 interstitial chiasmata remain at metaphase. It is then found that 

 when there is one chiasma in each arm this chiasma has been pushed 

 to the end. When there are two, the loop between them is smaller 

 than that containing the centromere. It seems that an equilibrium 

 is reached, characteristic for a particular type of bivalent with a 

 particular number of chiasmata, between two kinds of repulsion, the 

 generalised body repulsion which effects the even distribution of 

 chromosomes at all stages of mitosis and meiosis, and the localised 

 centromere repulsion, which effects the separation of the chromosomes 

 at anaphase in both mitosis and meiosis. The different degrees of 

 terminaUsation are then due to different degrees of centromere 

 repulsion, in relation to the length of the chromosome. Means of 

 testing these assumptions we shall discover later. 



(vii) Metaphase : the Structure of Bivalent Chromosomes. After 

 diakinesis the wall of the nucleus disappears and the spindle develops 

 as in ordinary mitosis. The bivalent chromosomes arrange 

 themselves on the equator with their pairs of centromeres 

 orientated just as the two daughter centromeres of a mitotic 

 chromosome are orientated in early anaphase. That is to say, they 

 lie symmetrically on either side of the equatorial plane and in an 

 axial direction from one another. Under favourable conditions of 

 fixation in the largest chromosomes they appear as single spherical 

 bodies a quarter of a micron in diameter {v. Ch. XII). Each 

 chromosome, i.e., pair of chromatids, now acts as though it had a 

 single centromere. The two centromeres of the bivalent (whose 

 repulsion has momentarily lapsed) begin to repel one another 

 actively, so that they are separated by the two segments joining 



