66 THE CHROMOSOMES 



(called chiasmata). If one of those chiasmata is 

 examined it will be seen that two out of the four 

 chromatids at this point form an X (Fig. ^d). These 

 chiasmata were seen as early as 1893 ^^* and are now 

 known to be an almost universal feature of the 

 diplotene and later stages in all organisms (certain 

 exceptions will be mentioned in the next chapter). 

 There is always (apart from very rare cases) at least 

 one chiasma in each bivalent and there may be as 

 many as twelve (in the long bivalent of the broad - 

 bean, Vicia Faba ^^^). As soon as the phenomenon 

 of genetical crossing-over was discovered it was 

 naturally suggested that the chiasmata formed the 

 physical basis of crossing-over. 



The average number of chiasmata in a bivalent is 

 known as the chiasma-frequency. One can speak of 

 the chiasma-frequency of a particular bivalent or of 

 the whole chromosome set in an organism. The 

 former varies from 1-0 to about 8-56 (in the long 

 bivalent of the domestic chicken ^^^). The genetical 

 significance of chiasma-frequencies is dealt with in 

 the next chapter. The appearance of bivalent s with 

 one and three chiasmata respectively is shown in 

 Fig. 9d and e. A bivalent with a single chiasma about 

 half-way along its length forms a four-armed struc- 

 ture while in the case of a bivalent with several 

 chiasmata there are a series of loops between the 

 chiasmata. It is important to distinguish between 

 true chiasmata and places where one chromosome of 

 which the bivalent is composed merely passes over 

 or under the other. ^^ The difference between these 

 two is shown in Fig. 9d ; in actual material it is 

 generally quite easy to tell them apart by careful 

 focusing up and down. 



There are two possible ways of interpreting chias- 

 mata (Fig. 13), and for a long time it was uncertain 

 which was correct. On the first hypothesis no 

 breaking of chromatids has taken place before the 

 appearance of the chiasmata, and the four threads are 



