Plate II: Figs. 1 and 2 — Metaphase II in plant homozygous for abnormal 10. Carmine 

 smear. Precocious poleward movement of neo-centric regions is clearly evident. One dyad 

 has a single neo-centric region (Fig. 4.5, dyad No. 8) while the left-most dyad has a neo- 

 centric region in both long arms (Fig. 4.5, dyad No. 7). This cell was figured in Rhoades 

 and Vilkomerson 1942. Figs. 3 and 4 — .Anaphase II in jilant homoz\gous for abnormal 10. 

 Carmine smear. Note that the rod-shaped monads with precocious neo-centromeres are 

 the first to reach the poles. Fig. 5 — Metaphase II in plant homozygous for abnormal 10. 

 Carmine smear. The only chromosome of the haploid complement which can be recognized 

 at metaphase II is chromosome 6 which has a satellite at the end of the short arm. In this 

 cell the chromosome 6 d\'ad is the second from the left. That the terminal chromosome of 

 the satellite is actuall\- a small knob is indicated by the formation of neo-centric regions al 

 the end of the short arm. Fig. 6 — Early anaphase II in plant heterozygous for abnormal 10. 

 Carmine smear. That the poleward movement of neo-centric regions is less rapid in hetero- 

 zygous than in homozygous abnormal 10 plants is indicated here by the relatively slight 

 attenuation of the rod-shaped monads. Fig. — 7 Late anaphase II in plant homozygous for 

 abnormal 10. Carmine smear. The previously greatlx- stretched rod monads with precocious 

 neo-centromeres have contracted. Note the inverted V-shaped chromatids. This is the same 

 cell shown in Figure 4.4. Fig. 8 — Side view of metaphase I in a normal plant showing the 

 fibrillar nature of the chromosomal fibers. Fi.xed in Benda, stained with haemotoxylin. 

 Paratiine section. The only chromosomal fibers j^resent are those formed by the true cen- 

 tromeres. Ordinarily chromosomal fibers are not evident in carmine smears since they are 

 destroyed by acetic-alcohol fi.xation and it is nccessar\' to use special techniques to demon- 

 strate them. Similar fibrillar chromosomal fibers are found at neo-centric regions when 

 proper fixation and staining methods are employed. Fig. 9 (top) — Polar view of meta- 

 phase I in normal plant. Fixed in Benda, stained with haemotoxylin. Parafhne section. 

 Note the arrangement of the ten bivalents on the ecjuatorial plate. This microsporocyte 

 was cut slightl}' above the metaphase plate. The next section, which includes the remaining 

 portion of this cell, is a cross section through the ten sets of chromosomal fibers. 



