THE MECHANICS OF THE CHROMOSOMES II5 



evidence in support of telosynapsis was adduced from the occurrence 

 of continuous rings of chromosomes at meiotic metaphase in some 

 organisms (chiefly Oenothera), but another more satisfactory explana- 

 tion can now be given for this (p. iii). 



Darlington was the first to suggest that if the parasynaptic interpre- 

 tation is adopted, all the differences between meiosis and mitosis can, 

 with the help of very few hypotheses, be deduced from the original 

 difference in the singleness or doubleness of the prophase threads. He 

 suggested that homologous chromosomes, or rather homologous 

 chromonemata, attract one another in pairs. This attraction is satisfied 

 in mitotic prophase, but not in early meiotic prophase, where the 

 chromosomes are at first single, and can only satisfy the attraction by 

 coming together in zygotene pairing. In meiosis, the spHtting of the 

 chromosomes, which occurs in the interphase before a mitotic division, 

 does not happen till pachytene when the chromosomes are already 

 associated in pairs. It therefore produces a set of four associated threads 

 and these fall apart into the diplotene loops, being held together by the 

 changes of partner at chiasmata. The falling apart of the four threads is 

 evidence that although two homologous chromosomes attract one 

 another, one pair of homologous chromonemata repels another similar 

 pair. 



Thus Darlington supposed that the spUtting of a chromosome in 

 preparation for the next mitotic division takes place in the interphase 

 before that division, and that the singleness of the meiotic chromo- 

 somes is due simply to the fact that they begin condensation and con- 

 traction for the division before the splitting has occurred. This hypo- 

 thesis is known as the Precocity Theory. 



A modification of the simple precocity theory has been proposed by 

 Huskins,^ who accepts Darhngton's hypotheses that the differences 

 between the two sorts of division are due to the repulsion between 

 pairs of chromonemata and attraction between single chromonemata, 

 but rejects Darlington's account of the origin of the singleness of the 

 meiotic chromosomes in prophase. According to Huskins, the splitting 

 of the chromosomes for one division takes place during the previous 

 division, forming a so-called tertiary split which causes the chromo- 

 some pairs at mitotic metaphase to be actually quadripartite, those at 

 meiotic metaphase octopartite bodies. During the interphase before 

 meiosis, this tertiary spUt must be supposed to be destroyed in some 

 way, only to be restored at pachytene; in about the diakinesis stage it 

 ^ Cf. Huskins and Smith 1934. 



