76 M. M. RHOADES 



1 and 2 of Plate II). It is not unreasonable to assume that dyads with both 

 homologous arms exhibiting neo-centromeres at metaphase II carried a knob 

 in each chromatid, while dyads with one neo-centromere consisted of one 

 knobbed and one knobless chromatid. Such heteromorphic dyads would arise 

 from heteromorphic bivalents by a crossover between the true centromere 

 and the knob. We believe that only knobbed chromatids have active neo- 

 centromeres at metaphase II, and that knobless ones are normal at this stage. 

 Unfortunately, knobs cannot be recognized at metaphase II, and the validity 

 of the above assumptions rests upon indirect but convincing evidence. 



Two types of disjoining monads are found at anaphase II, those which 

 are rod-shaped and those which are V-shaped. Monads which had one arm 

 extending poleward at metaphase II are rod-shaped. They are the first to 

 reach the pole. Indeed distal portions of such chromatids already had arrived 

 there during metaphase II owing to the early action of their neo-centromeres. 

 The V-shaped monads of anaphase II are derived from those chromatids 

 devoid of neo-centromeres at metaphase II. The poleward migration of some 

 monads is first begun by the chromosomal fibers emanating from the true 

 centric region, but shortly after anaphase is initiated chromosomal fibers 

 may arise from the ends of both arms. These terminally placed fibers, which 

 are directed to the same pole, propel their ends poleward with such rapidity 

 that the ends first overtake and then pass the centric region in the course of 

 anaphase migration. Consequently these monads reach the poles as inverted 

 V-shaped chromosomes (see Fig. 4.4). The spindle fibers from the true centric 

 region now are directed toward the spindle plate rather than to the pole — they 

 have reversed their orientation. This would be impossible if chromosomal 

 fibers were of a thread-like structure. It is more likely that these fibers repre- 

 sent nothing more than lines of force emanating from the centromere. In- 

 verted V-shaped chromatids are not invariably found at anaphase II. 



Some monads have chromosomal fibers only at the true centric region and 

 move poleward in a normal fashion. Either neo-centric regions are not pres- 

 ent, or else arise too late to be effective. It should be emphasized that a funda- 

 mental distinction exists between the rod and inverted V chromatids found 

 at anaphase II. The rod-shaped monads come from dyads with neo-centric 

 activity at metaphase II. Their supernumerary chromosomal fibers arise 

 from one arm. Their sub-terminal location suggests they may arise adjacent 

 to the knob, but this is merely a conjecture. The later-formed extra chromo- 

 somal fibers of the inverted V chromatids, which are knobless, are terminal 

 and arise from both arms. 



If a dyad is oriented on the spindle plate at metaphase II before the onset 

 of precocious neo-centromere activity, the supernumerary chromosomal 

 fibers arising from the knobbed arm of the chromatid situated slightly above 

 the spindle plate are directed toward the upper (nearest) pole, and those 

 from the bottom chromatid go to the lower pole — they are co-oriented (see 



