Il8 AN INTRODUCTION TO MODERN GENETICS 



appearances which are both on the limit of visibility, and it is hardly 

 possible to accept the evidence for the existence of half-chromatids as 

 firmly estabUshed. As yet investigators who criticize Darlington's 

 hypothesis of precocity on these lines neither agree among themselves 

 nor suggest a coherent alternative theory of the relation between the 

 two types of division. Even if it should eventually be established that 

 there are units smaller than the half-chromosomes which are usually 

 known as chromatids, it must be remembered that the precocity theory 

 is concerned only with the behaviour of these units in attracting and 

 repelling one another, and is not finally invaUdated if it turns out that 

 quarter-chromosomes behave as units in other respects, for instance, 

 in relation to X-ray breakage. 



2. Chiasmata 



Chiasmata appear first in diplotene as the "nodes" between the 

 loops into which the quadripartite chromosome pairs are opening out. 

 They are at first always interstitial, that is to say, located somewhere 

 along the length of the chromosomes, and not directly at the ends. 

 They may also be either distributed more or less at random along the 

 lengths of the chromosomes, the randomness being modified by inter- 

 ference (p. 125), or localized in particular regions. Their position, 

 however, does not remain constant: as diplotene and diakinesis pro- 

 gress, the chiasmata move towards one or both ends of the chromo- 

 somes, usually only towards the non-attachment end in chromosomes 

 which have a terminal attachment and towards both ends in chromo- 

 somes with an interstitial or median attachment. This movement is 

 spoken of as terminalization and occurs to different degrees in different 

 organisms. It is weakest in organisms with the longest chromosomes. 

 The terminaHzation leads not only to a concentration of chiasmata at 

 the ends of the chromosomes, which can be expressed as a terminaliza- 

 tion coefficient (i.e. ratio of terminal chiasmata to total number of chias- 

 mata) but also to an actual reduction in the total number of chiasmata. 

 This reduction might be due to the breakage of interstitial chiasmata, 

 but the evidence rather suggests that it is due to the pushing off of the 

 first chiasma to arrive at the end of a chromosome by the second 

 chiasma which becomes terminalized to that end later (see p. 127). 



The terminaUzation of a chiasma can be arrested if there is a change 

 of homology along the length of a chromosome, and the unusual 

 failure of terminalization may thus give evidence of a dissimilarity 

 between chromosomes, other parts of which may be able to pair. 



