CHROMOSOMES OF MOSQUITO 529 



phase plate approximates biradial symmetry, the transverse and 

 parallel axes each dividing symmetrical halves. 



Figure 17 is the same stage. The shortness of the chromo- 

 somes is due in part to sloping of the polar axis, in part to fixa- 

 tion. In figure 18 the shortness of the chromosomes is also due 

 to shrinkage, illustrating the effect of imperfect technique. The 

 figure shows that the small pair may occupy the centre of the 

 metaphase plate. In figure 19, a transverse optical section of 

 the chromosomes in the same stage, the members of each pair 

 are closely approximated, apparently a matter of fixation. Fig- 

 ures 17, 18, and 19 are all from the same testis which showed 

 imperfect fixation. The small pair in figure 19 is on the left, 

 as may be judged by focusing, although the diameter is as great 

 here as that of the others. 



Figure 20 shows that the chromosomes begin to split in the 

 middle. The plate is so bent that, although the transverse axis 

 is in the plane of the paper, five of the chromosomes show the 

 beginning of the split. Figure 21 is a similar stage. The 

 chromosome at the right has been pulled out of position by sec- 

 tioning and broken at the point of attachment of the spindle 

 fibres; the daughter chromosomes are somewhat separated. 

 Figure 22 is a side view of a chromosome from another plate at 

 this same stage and figure 23 is a transverse optical section of a 

 similar plate The ends of the chromosomes seen at the left and 

 right are visible on account of bending. Figure 24 is a side view 

 of a chromosome in a little later stage of division showing part 

 of another chromosome that was in the same optical section. 

 Figure 25 is an oblique view of the chromosomes at a slightly 

 later stage, 



3. Anaphase and telophase. Figure 26 is an optical section of 

 the ends of the chromosomes just separated in anaphase. Figure 

 27 shows a later anaphase with the chromosomes still displaying 

 some of their parallel arrangement. 



The spermatogonial chromosomes always seem to pull out very 

 thin in anaphase and the members of the homologous pairs are 

 often so close as to be inseparable. In figure 27 the members of 

 the small pair have apparently fused completely. 



