256 CHROMOSOMES IN HEREDITY: MECHANICAE 



to somatic pairing (q.v.) and persist in this relationship until they 

 pair. Apart from such exceptions it would therefore be expected 

 that the chromosomes in pairing would often interlock, a strange 

 chromosome passing through a loop between associations of the 

 two homologues. Necessarily, this would be detectable only in 

 favourable material. Interlocking has been seen taking place at 

 zygotene in Dendrocoelum (Gelei, 1921), Viviparus (Belar, 1928 h) 

 and Allium (Levan, 1933 c) and maintained at metaphase, in 

 organisms with and without polarisation of the zygotene nucleus 



Proximal 



Proximal -l^i si's/. 



Proximal 



Oislal 





1 

 ^ 





TlVO Pi/7^'S. 



One Ping. 



Fig. 83. — Chromatid diagram of diplotene and metaphase configura- 

 tions showing interlocking of pairs of ring-bivalents (left) and 

 within a ring-of-four (right) before and after terminalisation. 



{e.g.f Stenohothrus and Hyacinthus), and in organisms with all 

 degrees of terminalisation. 



The detection and analysis of interlocking at metaphase is of 

 importance, for the form it assumes will distinguish between various 

 theories of chromatid relationship in organisms with and without 

 terminalisation. On the hypothesis of chiasmatype crossing-over 

 the chromatids derived from one parental chromosome fall apart 

 from those of the other at diplotene so that an interlocked chromo- 

 some will lie between the separated chromosomes in the diplotene 

 loops. 



Simple interlocking is classifiable into three types at metaphase 

 in an organism with terminalisation, according to whether the 

 chiasmata, formed by the interlocked chromosomes, are moving 



