MEIOSIS 



269 



tion, the opening is along the synaptic plane in some portions of the tetrad 

 and along the equational plane in other portions. Where two openings 

 in different planes meet, the chromatids must "trade partners." When 

 viewed from a given direction under the microscope two of the chromatids 

 may appear to cross each other at the resulting chiasma while the other 

 two continue more or less straight, and they are usually represented thus 

 in conventional diagrams; but a three-dimensional model makes it clear 



"■>;x_- 



B 



y^ 





T 



""--'*v«^-/' 



D 



Fig. 158. — A, tetrad with three chiasmata (abc) as interpreted on the two-plane theory. 

 Two of the chiasmata are shown yielding chromatid exchanges (a'b'), while the third (c) 

 separates in anaphase (third figure) without exchange. B, similar tetrad as interpreted 

 on the one-plane theory, with terminalization: chiasma a has been terminalized to a', b has 

 been moved to b', and c has been terminalized to c'. C, D, tetrads disjoining without 

 chromatid exchange (two-plane theory). Spindle-attachment regions indicated by 

 stippling. 



that all of the chromatids may be equally bent, the angle of view determin- 

 ing which two appear to be crossed. If, however, there is a twist of 

 the tetrad at the chiasma, two chromatids may be crossed and in contact 

 while the other two are not. When opening out begins at a number of 

 points, a diplonema tetrad may appear as a series of rings in alternating 

 vertical and horizontal planes (Fig. 158, A). 



The fate of the chiasma as meiosis proceeds beyond the diplonema 

 stage may be variously conceived on the basis of the two-plane theory. 

 The four chromatids might contract to the diakinesis condition and move 

 apart in the first anaphase without undergoing any exchange of portions 

 (Fig. 158, C, D). On the contrary, the continued opening out of the 

 tetrad in the diplonema stage and early diakinesis might impose such 



