528 



CELL MECHANICS 



and the connection between the two chromosomes may become so fine 

 as to be invisible. Such cases have sometimes been misinterpreted 

 as due to failure of pairing or precocious separation of a particular 

 pair (D., 1936^, on Chorthippus, cf. Belar, 1929 a, on Stenobothrus, 

 Larter, 1932, on Ranunculus, D., 1933, on Secale ; cf. also " distance 

 conjugation " in the Hepaticae). 



The property of the centromeres of bivalents which leads to their 

 relative orientation is revealed by the behaviour of univalents. It 

 might be supposed that univalents at meiosis would behave like 



Fig. 152. — Positions of the centromere in Tradescantia {zx) in 

 relation to the spindle. Rod chromosomes are on the periphery 

 and lie outside their centromeres owing to body-repulsions. 

 All pairs of centromeres are equidistant. X 2,400. 



chromosomes at mitosis, but they do not do so until late metaphase 

 or the beginning of anaphase. They are delayed in their reaction to 

 the spindle, as we should expect from the precocity of the prophase 

 of meiosis. And, as we should expect also, the delay affects all their 

 centric reactions — congression, orientation and division. The ques- 

 tion, then, is what change the centromeres undergo between the 

 time when they can be orientated in pairs but not singly (as in 

 bivalents and, at early meiotic metaphase, in univalents), and the 

 time when they can be orientated in pairs but cannot be orientated 

 singly (as in pairs of chromosomes with pseudo-chiasmata at 

 mitotic metaphases and in univalents at early first meiotic anaphase). 

 Clearly it is a change which permits one body to behave like two, 



