PAIRING AND COILING 93 



Probably, therefore, in Fritillaria, and many similar organisms that 

 will be described later, pairing begins near the centromere and 

 passes, sometimes regularly and sometimes intermittently, along 

 the chromosomes until it is complete or until it is interrupted. 



The unstable stage of pairing is called zygotene. The stable 

 stage when pairing has ceased is pachytene. Threads that are still 

 unpaired are then seen to be double, although the paired threads 

 are still single (D., 1934 a, Zea, and 1935 a, Fritillaria). It is there- 

 fore possible that when delayed the chromosomes divide before 

 they can pair, and their division inhibits their pairing. 



At leptotene the chromosomes are contorted in a way that 

 probably corresponds to the relic spiral of the prophase of mitosis. 

 The chromomeres are seen to be distributed at unequal distances 

 along the chromosome thread, and the chromosomes as they pair 

 are seen to correspond in regard to this structure. The constant 

 linear arrangement of the chromosomes already suggested by the 

 constant position of centromeres and nucleolar organisers is now 

 established by the characteristic size and position of every chromo- 

 mere. At pachytene successive granules are more closely crowded 

 along the thread. This may be due to their staining more deeply, 

 since it cannot be due to a linear contraction. Sometimes the 

 threads are longer at the end of pachytene than at the beginning 

 (Belling, 1931, on Lilium ; Roller, 1936, on various marsupials). 

 In Phrynotettix magnus Wenrich (1916) was able to represent each 

 of the paired chromosomes separately, showing that they were 

 present in the haploid number and that corresponding types in 

 regard to the granular structure appeared in different nuclei. In 

 Phrynotettix the chromosomes were already contracted at this stage 

 to not more than three times the length characteristic of the somatic 

 metaphase. But in plants they are still five or six times their 

 mitotic metaphase length in the few species in which the contents 

 of the whole nucleus have been separately drawn (Belling on Lilium 

 and Aloe, 1928 d ; Dark on Bellevalia, 1934, and Moffett on 

 Anemone, 1932, Fig. 23, v. Table 3). 



During the pachytene stage, which may be almost indefinitely 

 prolonged without perceptible change, the chromosomes become 

 coiled round one another. This relational coiling is probably 



