06 KANSAS UNIVERSITY QUARTERLY. 



from the accessory chromosome, which, on account of its greater 

 density and its nearly spherical form, could be easily observed up to 

 this time. 



Since the pseudo-germinal-vesicle stage, the accessory chromosome 

 has undergone several interesting changes. As we have seen, it is 

 then remarkable for its large size and strong attinity for chromatin 

 stains, In the active prophase following this, while the chromatin is 

 collecting in diffuse, flaky masses preparatory to tetrad formation, 

 this element decreases much in size, and at the same time its outline 

 becomes very irregular. In a later stage, shown in figs. 5, 9, the con- 

 tour of this element has again become regular; it has resumed its 

 spherical form, and has diminished in size, until now it is no larger 

 than a chromosome in the metaphase of the first spermatocyte division. 

 It goes to the equatorial plate without previously showing any sign 

 of division. 



I can see but one plausible explanation for the behavior of the ac- 

 cessory chromosome in the pseudo-germinal-vesicle stage and in the 

 late prophase succeeding. While the nucleus is in the ditfuse condi- 

 tion, it would seem that the accessory chromosome acts as a store- 

 house for practically all of the chromatin of the cell, or, more 

 properly, serves as a center around which the chromatin becomes 

 condensed. As has been mentioned before, the later behavior is very 

 similar to that of the germinal spot in some eggs. 



With the disappearance of the nuclear membrane, mantle fibers are 

 seen connecting each centrosome with the chromosomes. (Fig. 11.) 

 The chromosomes heretofore distributed irregularly throughout the 

 nuclear space are drawn into the equatorial plate, and a very short 

 spindle is formed. (Fig. 13.) 



Later the centrosomes move apart some distance, until the cell pre- 

 sents the appearance represented in fig. 15. By this movement the 

 shape of the chromosomes is altered somewhat, and they now show 

 signs of division. At this stage the astral rays can be seen, connect- 

 ing the centrosomes with the cell membrane. They are very distinct, 

 and there is no question but what they really extend from the centro- 

 some to the cell membrane. From their separation upon the nuclear 

 membrane, up to this time, the centrosomes have been of an irregular 

 spherical form. Now, however, their shape is changed, and they ap- 

 pear as rather large, cone-shaped masses, with their apices turned 

 toward the equatorial plate. (Figs. 15, 16.) At no time is the out- 

 line of the centrosome exactly spherical. 



When the cell has reached this stage a very unusual thing occurs. 

 By the con-traction of the astral rays attached to the cell membrane 

 the centrosomes are drawn still farther apart, and take up their i^osi- 

 tion at points a short distance from the cell membrane. (Fig. 16.) 



