28O ELIZABETH A. SMITH. 



Usually, in a polar view, the chromosomes appear bipartite 

 or round because the cut does not exactly halve the bivalents. 

 The sex-chromosome in such a stage of nuclear change is round 

 and shows no evidence of a constriction. Figs. 42, 43 and 44 

 are polar views of the twelve bivalents and the single univalent 

 sex-chromosome. 



In Sympetrum there is usually an outer ring of seven bivalents 

 and the sex-chromosome, surrounding five in the center. The 

 chromosomes vary in size somewhat, although five are of nearly 

 uniform size; one is larger and the others grade down gradually 

 to the smallest which is smaller than the sex-chromosome. In 

 some cases the latter may appear to one side, and then only 

 four bivalents occur in the middle, one of the central bivalents 

 apparently replacing the sex-chromosome in the outer ring. 

 In a side view of this stage, all the autosomes are compact bi- 

 valents with four shortened arms and a central clearer area. 

 The central bivalents can be seen below the others in an exact 

 metaphase. The sex-chromosome can be easily distinguished from 

 the bivalents by its rounded appearance and clear vesicle-like 

 zone around it. All are connected with double spindle threads. 

 The centrosomes are extremely large. The bivalents when 

 dividing pull into two halves, presumably along the line of 

 junction of the two ends of the autosomes which agrees with 

 McClung's statement that separation between the parts of 

 bivalent chromosomes is more likely to occur along spaces be- 

 tween whole chromosomes. 



The bivalents do not seem to behave like tetrads when divid- 

 ing. Occasionally they pull out into the form depicted in Fig. 

 48, but when the telophase is examined the autosomes show 

 no split. If true tetrads were formed the telophase number of 

 autosomes would be doubled or each autosome would be bipartite. 

 Fig. 49 shows the sex-chromosome lagging behind the others 

 and in Fig. 50 it is dividing after the others are well on their 

 way to the poles. Figs. 51 and 52 are telophases of this division. 

 Usually in later stages of division the sex-chromosome cannot 

 be distinguished from the autosomes, but it occasionally stands 

 out in a polar view of a telophase as in Fig. 53. In the telo- 

 phase, the chromatin is massed at each pole, the cytoplasm 



