K. — BOTANY. 177 



components. It has been observed in many forms that the bivalent struc- 

 ture has tlie appearance of a twisted double thread. Ah-eady in 1909 

 cytological study oi the salamander had led Janssen ' to conclude 

 that fusion might take place at the crossing points, so that when the 

 twin members ultimately draw apart each is composed of alternate 

 portions of the original pair. Morgan explains the breeding results 

 obtained with Drosophila by a somewhat similar hypothesis. He also 

 concei\"es that in the process of separation of the twin lengths of 

 chromatin cleavage between the two is not always clean, portions of 

 the ono becoming interchanged with corresponding segments of the 

 other, so that both daughter chromosomes are made up of comple- 

 mentary sections of the maternal and paternal members of the duplex 

 chromosome. To picture this let us imagine that two bars of that 

 delectable substance, Turkish Delight, one pink and one white, are laid 

 alongside and are then given a half twist round each other and pressed 

 together. If, with a knife inserted between the two pieces at one 

 end, the double bar is now sliced longitudinally down the middle neither 

 of the two halves will be wholly pink or wholly white. Each half will be 

 pai-ticoloured, the pink portion in one and the portion which is white in 

 the other representing corresponding regions of the original bars. If 

 the complete twist is made, or if the number of turns is still further 

 increased before the slicing, the number of alternately coloured por- 

 tions will naturally be increased coi'respondingly. Though the precise 

 manner in which the postulated chromosomal interchange is brought 

 about in Janssen 's ' chiasmatype ' and Morgan's ' crossing-over ' 

 scheme is different, the resulting gametic output would be the same. 

 A critical examination by Wilson and Morgan," from different aspects, 

 of Janssen 's interpr-etation of the cytological evidence including dis- 

 cussion of his latest suggestion that in the case of compound ring- 

 chromosomes cleavage in one plane would result in the separation of 

 homologous elements in one ring but not in another has just appeared. 

 These authors are not disposed to accept Janssen 's conclusions,^" but 

 reserve their final statement pending the appearance of his promised 

 further contribution. Should Janssen 's view of the evolutions of these 

 complex chromosome structures be upheld, the process of segregation 

 might in such cases become extended over more than one mitosis, as 

 on the reduplication theory is conceived to be the case at some point, 

 though evidence in this direction has hitherto been lacking. Bisection 



'ft 



of a bivalent chromosome in this fashion might, moreover, yield the 

 class of results to explain which Morgan has found it necessary to 

 have recourse to hypothetical lethal factors. On the main issue, how- 

 ever, both schemes are in accord. A physical basis for the phenomenon 

 of linkage is found in the presumed nature and behaviour of the 

 chromosomes, viz. : their colloidal consistency, their adhesion after 

 pairing at the points of contact, when in the twisted condition, and 

 their consequent failure to separate cleanly before undergoing the 

 succeeding division. 



* La Cellule, xxv. 

 » A7n. Nat., vol. 54, 1920. 

 '" See Com-piex Rendiis Sloe. Relg. Biol.. 1919 

 1920 N 



