MAJOR AND MINOR SPIRALS 485 



homologues coiling in the same direction. Is this the same spiral 

 as that determining the internal spiral ? 



(iv) If so, why does it determine a relational spiral at one time 

 and an internal spiral at another ? 



(v) Finally, why do the chromatids not coil jointly instead of 

 separately during the prophase, making use of the relic spirals 

 from the preceding division instead of undoing them ? 



New work on the coiling of the chromosomes at meiosis now 

 enables us to answer some of these questions. 



(ii) The Meiotic Cycle. An unpaired chromosome at the first 

 metaphase of meiosis consists, like a mitotic chromosome, of two 

 chromatids lying side by side with an undivided centromere. Two 

 paired chromosomes differ merely in having an exchange of partner 

 amongst their chromatids. Meiotic chromosomes, however, are dis- 

 tinguished from mitotic ones slightly but definitely in their super- 

 ficial properties : on the one hand they are shorter and broader and 

 on the other they are more regularly and closely associated. These 

 differences depend on their internal structure. 



The large meiotic chromosomes that have been accurately 

 examined after appropriate treatment have been shown to consist 

 of a minor spiral, coiled into a major spiral. The minor spiral corre- 

 sponding in diameter to the mitotic internal spiral, was first sug- 

 gested by Fujii and demonstrated experimentally by Kuwada and 

 Nakamura. The secondary coiling accounts for the greater reduc- 

 tion in length of meiotic chromosomes. Moreover, the nucleolar 

 constrictions are apparently constrictions of the minor spiral since 

 they are invisible in a thread coiled into a major spiral as well. 

 Kuwada (1935) supposes that this develops after the primary minor 

 spiral. They may alternatively be supposed to develop side by side, 

 since in this way variations in the degree of linear contraction would 

 require the minimum of variation in the angle of the spiral. 



This difference in spiralisation between meiosis and mitosis is 

 generally correlated with precocity of the prophase. It is accom- 

 panied by a second difference. The two chromatids are sometimes 

 jointly coiled both as to their major and perhaps their minor spirals. 

 They then come apart at late metaphase or anaphase (Kuwada and 

 Nakamura, 1933 ; Sax and Humphrey, 1934). Such seems to be 



