SYNAPSIS AND CHROMOSOME ORGANIZATION 489 



while the longer arms have become free. From the standpoint of 

 simple mechanics it would be difficult to understand how a 

 tetrad such as is seen in cell C, for example, could give rise to 

 the condition in cell G. It would be natural to suppose that the 

 crossed longer arms (at the left) would require a greater amount 

 of force for separation than the shorter arms in contact at the 

 right; the pull of the spindle fibers would be expected to be 

 exerted more strongly on the shorter ends. The conditions in 

 cell G are the exception rather than the rule, and I have not found 

 an earlier tetrad which showed the longer arms free as they are 

 in this cell. However, in figure 19, plate 3, there is a ring- 

 shaped tetrad which, by the mere separation of the lightly at- 

 tached ends of the long arms at the left, would pro\dde a condition 

 analogous to that in figure 20, which resembles those in cell G, 

 plate 2. A condition which is the reverse of that in figure 20 

 is shown in figure 16, where the short arms are free. Such a 

 shape could readily arise from that in figure 15 by a little more 

 separation along the plane between the arms at the right and 

 their flexure at the point of spindle-fiber attachment. 



It might be remarked that one cannot be certain whether 

 chromosome 11 or 12 is represented in figures 15 and 17 because 

 the synaptic points are not indicated by clefts as they are 

 in figure 19. Figure 16 is doubtless number 12, with its great 

 disproportion between arm lengths, while figure 20 is more 

 probably number 11. Figure 18 is problematical. 



In cells E and G chromosome 7 (plate 2) seems to have the 

 shorter arms united and the longer arms free, while in F the 

 longer arms are attached to each other and in H they have 

 been the last to separate. In cell E the difference between arm 

 lengths seems much less than in the other cells. This is to be 

 accounted for by the foreshortening of the longer (free) arms 

 and by the stretched conditions of the attached shorter arms, 

 shown by their smaller diameter. 



As a net result of such observations one may conclude that the 

 movements of the constituent chromatids of a tetrad are sub- 

 ject to great variation and that with respect to the separation 

 of the ends, there is sufficient e\'idence to show that either the 



