xiv] CROSSING-OVER 225 



The explanation of these facts offered by Prof. MORGAN 

 is of this kind. He assumes that when the chromosomes 

 undergo side-to-side pairing in the zygotene stage, they do not 

 simply lie parallel with each other, but become more or less 

 twisted round each other like the strands in a rope, and that 

 when the split takes place which separates the chromosomes 

 in the first maturation (heterotype) division, the chromo- 

 somes do not unwind themselves, but the split is straight, 

 so that part of what was one chromosome remains attached 

 to the other, while a corresponding . part of the second 

 becomes attached to the first. Such twisting of the zygo- 

 tene chromosomes has been observed in several animals and 

 plants, and while some observers maintain that they be- 

 come untwisted as they shorten to form the heterotype 

 "gemini," others (notably JANSSENS, who worked with 

 Amphibia) describe a process exactly corresponding with 

 what MORGAN'S hypothesis demands (cf. Text-fig. 13, 

 p. 90). It is unfortunate, however, that hitherto nothing 

 of the sort has been discovered in Drosophila itself, perhaps 

 in consequence of its unsuitability for cytological study. 



MORGAN thus assumes, firstly that when chromosome A 

 pairs longitudinally in synapsis with its homologue a, they 

 become twisted round or across each other, and that instead 

 of separating completely, a part of A remains attached to a 

 and an exactly corresponding part of a takes its place in A. 

 Secondly, he assumes that the factors for the coupled 

 characters are arranged in a definite order along the chromo- 

 somes like beads on a string; these factors in chromosome A 

 may be lettered Af, N 9 0... and in its homologue a the 



which the above imaginary example is based, the average ratio of crossing-over 

 between factors A and B was i per cent., between B and C 29*7 per cent, and 

 between C and D 3 per cent. The percentage of crossing-over between A and 

 C was found to be 32'2, between B and D 33*7, and between A and D 35*5. 



D. C. 15 



