Il6 AN INTRODUCTION TO MODERN GENETICS 



is succeeded by another split for the division which follows the second 

 meiotic division. 



The two theories give quite different accounts of anaphase separa- 

 tion. According to Darlington, the initial separation of the chromo- 

 somes in anaphase is in mitosis due to the division of the centromeres 

 at this stage and in meiosis to an increased repulsion between the 

 centromeres which now begins to overcome the forces holding the 

 chromosomes together at the chiasmata. According to Huskins, on the 

 other hand, the separation of the mitotic chromosomes is analogous to 

 the formation of diplotene loops ; it is a consequence of the formation 

 of the tertiary split which converts the paired mitotic chromosomes 

 into a pair of mutually repelling paired threads. Huskins claims that 

 the centromeres are double throughout the mitotic prophase, not 

 remaining single until metaphase as Darlington supposes. For the 

 meiotic metaphase separation Huskins invokes the same mechanism as 

 does DarUngton; the tertiary split occurring in late diakinesis has no 

 consequences until two divisions later. 



It may appear that a decision between the two theories could easily 

 be taken by a simple observation of the single or double nature of the 

 chromosome thread at relevant stages. Indeed, such observation is the 

 main basis on which Huskins advances his theory. Darlington points 

 out, however, that the observations which have been published of the 

 tertiary spUt are highly contradictory among themselves, and suggests 

 that the diameter of the chromosome spiral is sufficiently near the 

 wave-length of ordinary hght for optical illusion to play a large part in 

 determining the microscopical appearance of the thread. Huskins 

 retorts that, while it might be possible to mistake a single cylinder for 

 a double thread when observing it from the side, an end-on view 

 which shows a double thread can hardly be in error.^ 



The question has, however, recently been investigated by rays of 

 wave-length very short compared with the structures involved, namely 

 X-rays. Nuclei in the resting stage are X-rayed and examined for 

 breakages in the next mitotic metaphase.^ If at the time of radiation the 

 chromosomes are single, breakages (with rejoining of the ends) will 

 give dicentric and acentric fragments (i.e. with two or with no centro- 

 meres) each consisting of two chromatids formed by spUtting after the 

 operation. If on the other hand the chromosomes are already double, 

 pseudo-chiasmata will arise. It has been found that the critical period 



^ Rev, Kaufman 1936. 



2 Mather and Stone 1933, Riley 1936, Mather 1937a. 



