MEIOSIS 



259 



The pachynema now passes into the diplonema stage, so called because 

 doubleness in the thick threads, if hitherto obscure, now becomes plainly 

 visible (Figs. 150, 4.; 151, d; 155, a). In each tetrad two of the chromatids 

 widen out from the other two through the greater portion of their length, 

 but all four tend to remain together in a characteristic manner in one 

 or more regions. In passing along a tetrad it is often observed that at 

 such a region the paired chromatids appear to exchange partners, thus 



Fig. 152. — Chromosomes of Zca Mays in prophase of first meiotic division in micro- 

 sporocyte. a, whole nucleus, showing all of the chromosomes (in closely synapsed pairs). 

 Chromosome III at upper right; chromosome VI attached to right side of nucleolus; 

 (c/. Fig. 65); looped over VI is chromosome I. b, chromosome VII; note knobs, c, 

 chromosome VIII. d, chromosome III. e, chromosome IX. /, chromosome X. Spindle- 

 attachment regions indicated by arrows. Taken from different preparations and therefore 

 not strictly comparable as to size. Cf. Figs. 66 and 170. {Photographs by McClintock.) 



constituting a chiasma. The origin and fate of these chiasmata are 

 matters of cardinal importance with respect to the significance of meiosis, 

 hence they will be discussed in detail in a later special section. During 

 this stage the threads often appear markedly twisted {strepsinema 

 condition). It should be noted that a diplonema tetrad which has 

 several chiasmata, and therefore several successive closed or open loops 

 in alternating vertical and horizontal planes, may present a false appear- 

 ance of twisting when viewed under the microscope. During the 



