CHROMOSOMES AND HEREDITY — DARLINGTON 421 



threads, and at the same time fall apart. But when they do so they 

 stick at certain points, as Janssens had said, the chiasmata. 



Why do they fall apart, in one sense, and stick together, in another? 

 Their structure gave the answer. Contrary to Janssens's view, the 

 chiasmata always had the same structure: tliey were exchanges of 

 partner between half-chromosome threads, chromatids as we call them. 

 Further, these exchanges could be shown, on internal, cellular, micro- 

 scopic, evidence to be invariably connected with a previous crossing- 

 over between chromatids of the partner chromosomes. On the 

 simplest assumption, therefore, chiasmata were determined by such 

 crossing-over. The arrangement of four chromatids could be shown 

 in a diagram that was at once genetic and cytological in its implica- 

 tions. With capital and small letters in sequence for the pairing 

 chromosomes, an asterisk for the mechanical center, and dots for the 

 points of breakage, the diagram would be as follows : 



Thus from two chromosomes, ABCDEF and dbcdef^ two new 

 chromatids, ABcdef and abCDEF^ had been formed, and the four 

 chromatids would pass as chromosomes into the four germ cells 

 formed by meiosis. The existence of these four cells would, as 

 Janssens had put it, be justified by each of them being a unique com- 

 bination of available genes different from the rest. 



One could not of course prove that this principle was universally 

 true (in those days most biologists believed that propositions ought 

 to be "proved") ; one could merely hope to render it increasingly prob- 

 able. This hope was gradually realized. The critical configurations 

 of several chromosomes united by, or interlocked with, successive 

 chiasmata, the comparisons of frequencies and distributions of 

 chiasmata and crossing-over in different organisms, in polyploids, in 

 hybrids, with inversions and interchanges of segments of chromosome, 

 in plants with defects of chromosome pairing and of sexual reproduc- 



