488 D. H. WENRICH 



plished in the anaphase may be well advanced before the ele- 

 ment reaches the metaphase, while in other cases these move- 

 ments are largely deferred until after the element has been 

 oriented in the metaphase spindle. 



There are some pronounced differences in shape of the atelo- 

 mitic chromosomes in the metaphase. In the case of chromo- 

 some 12, for example, a different shape is seen in each of the 

 cells, E, F, G, and H. It is easy to see how a simple move- 

 ment of the oppositely directed dyads in cell E toward their 

 respective poles would give rise to the condition in cell H ; but 

 no such simple movement would account for the shapes in F and 

 G. The explanation of these various shapes, I think, may lie in 

 the statement made above as to the variations in chromatid 

 movement in the earlier stages. A search among the tetrad 

 figures of these earlier stages should reveal the forerunners of the 

 shapes seen in metaphase. In cell C, for example, is a condition 

 of chromosome 12 which would readily give rise to that in cell 

 E if the shorter arms of the V's are toward the right and the 

 apices of the V's are pulled out toward opposite poles. Simi- 

 larly for chi'omosome 11, the tetrad in cell C might easily take 

 its place in the spindle in the form shown in cell E and the 

 tetrad of number 10 in cell D could, by orientation, assume the 

 shape in cell E. In cell D, on the other hand, occurs a condi- 

 tion of number 12 whic-h in many respects is similar to that in 

 cell F. The chief difference is that in the latter the short arms 

 of the V's have been reflexed toward the longer arms. The 

 tendency for the shorter arms to bend back in this way 

 when they are free is not infrequent among the atelomitic 

 chromosomes, as shown by McClung ('14). A somewhat similar 

 metaphase of number 11 or 12 from another cell is shown in figure 

 18, plate 3. Figure 17 shows an earlier tetrad which could give 

 rise to it by the separation of the ends on the right and their 

 flexure backwards toward the left. 



None of the earlier stages for chromosome 11 or 12 as seen on 

 plate 2 indicate a condition which could by any simple move- 

 ment give rise to the shapes seen in cell G. Here the curious 

 condition is found of the shorter arms remaining in contact, 



