pinney: chromosomes of anasa tristis. 355 



All of the chromosomes lie with their long axis in the plane 

 of the equatorial plate, and from a study of lateral views of 

 this stage it is evident that the plane of the split in the chromo- 

 somes coincides with the same plane. Ideal conditions for equal 

 division of chromatin are therefore present. Division is simul- 

 taneous throughout the group. The separated halves move 

 toward opposite poles, maintaining their relative positions un- 

 til the pole is nearly reached. Thus the shape of the spindle 

 during anaphase is altered to accommodate the varying condi- 

 tions. See figures 12, 13 and 14. There is no evidence as to 

 the method of spindle-fiber attachment, and it is doubtful 

 whether the matter can be decided in this material. Figure 15 

 is a drawing of a section through an entire cyst. Most of the 

 cells are in metaphase. A few are in anaphase. The polarity 

 exhibited in this section is typical of all cells of this genera- 

 tion. 



In lateral views of anaphases in young cells the chromosomes 

 are distinct. In the older, more crowded cysts the tendency of 

 the individual elements to coalesce produces the appearance of 

 a solid band of chromatin in the equatorial plate. Later stages 

 of the same show two separate bands with spindle fibers be- 

 tween. The plate arrangement is lost when the pole is reached. 

 Figure 17 shows a polar view of an early telophase. The 

 chromosomes have not yet lost their identity, although they 

 have apparently begun to disintegrate. Between the telophase 

 and the succeeding metaphase there is an increase in the amount 

 of chromatin. The formation of a nuclear membrane and the 

 further dissolution of the chromosomes follow. A typical telo- 

 phase from a young cyst is shown in figure 18. The daughter 

 nuclei are flattened, but assume a spherical outline as the diffu- 

 sion of chromatin proceeds. Figures 19 and 20 show the cul- 

 mination of this process. 



The inauguration of the next mitotic cycle is marked by the 

 appearance of an exceedingly long and much coiled spireme, 

 whether continuous or not it is impossible to say. At any rate, 

 after the thread has thickened and shortened considerably it is 

 plainly seen to exist in segments which finally condense to form 

 the homogeneous bodies of the chromosomes. Figures 21 to 

 28, inclusive, show the successive steps in this transformation. 

 The shortening segments show various twists, bends and con- 

 tortions. See figures 22, 23, 24 and 25. Not until very late in 

 the prophase does the longitudinal split in the threads become 



