pin^ey: chromosomes of anasa tristis. 363 



has shown conclusively that the first division plane is longi- 

 tudinal. However, in this case there is no positive evidence to 

 offer. The chromosomes enter the equatorial plate with their 

 longitudinal axes parallel to the spindle fibers. Figures 16 and 

 17, plate LXVIII, show that the polar ends of the chromosomes 

 converge toward the poles of the spindle. That both methods 

 of division may be employed by different members of the group 

 is shown by a study of the accessory at this stage. This ele- 

 ment, which is composed of two chromatids, was last observed 

 in the metaphase lying outside of the ring. The element (x) 

 in figure 16, plate LXVIII, is undoubtedly the accessory chro- 

 mosome. This drawing is not limited to one plane. The small 

 chromosome is shown dividing in its usual manner. This ele- 

 ment, it will be remembered, is located at the center of the 

 ring and no element of the ring could lie so far from this center 

 as the one marked (x) . This, then, must lie on the outside of 

 the circle. From the arrangement found in polar views of the 

 metaphase one would expect in the lateral views to find sec- 

 tions which would show four chromosomes in the same plane. 

 These would include the small central member, two chromo- 

 somes of the ring, one on either side, and, at one side, the 

 accessory. Such a section is shown in figure 17, plate LXVIII. 

 The small chromosome, it will be seen, occupies the axis of 

 the spindle, which in this view appears asymmetrical, the 

 asymmetry being due to the extra divergence of the fibers at- 

 tached to the accessory. 



In these lateral views of anaphases the structure of the ac- 

 cessory is easily determined. It is composed of two chroma- 

 tids, which are shown in figures 19, 20 and 21, plate LXVIII. 

 All of the other chromosomes contain four chromatids, 

 although in some only two of the four appear in the same 

 plane. The accessory is easily identified by its form in these 

 stages, where it is shown to divide tardily, division not be- 

 ginning until the ordinary chromosomes have entirely divided. 

 In their journey to the poles the accessory chromatids lag be- 

 hind the others. Figures 17 to 26, plate LXVIII, show this 

 behavior at different stages. All of the chromosomes of the 

 complex keep their relative positions until the pole is reached. 

 Figures 1, 2, 3 and 9, plate LXIX, show polar views of the 

 dividing groups with the inner and outer members in their 

 characteristic positions. As soon as the pole is reached the 



2-Univ. Sci. Bull.. Vol. V, No. 20. 



