386 EDMUND B. WILSON 



in many other cases it very clearly shows a double series of vari- 

 cosities that are accurately paired in the two longitudinal halves, 

 as if the rod originally consisted of a series of large granules or 

 segments that afterwards underwent longitudinal fission. Some 

 of the various forms that appear are represented in fig. 100. 

 Of these forms one is far more frequent than any of the others — 

 that which shows three pairs of segments (100 d), which in the 

 best cases are very sharply marked, in others less distinct though 

 evident, in others barely perceptible. In some cases (usually 

 more elongate forms), four segments are apparent (fig. 100, c), 

 but no case has been seen with more than four. In a few cases, 

 where the rod seems to be shorter, less than three segments appear, 

 and an almost quadripartite form results (fig. 100 e). Some of 

 these cases are obviously due to a sharp curvature of the rod, so 

 that in foreshortened view only the end segments are seen; but 

 I have seen a few cases in which the rod seems to have simply 

 shortened and two pairs of the segments seem to have fused to- 

 gether. In fig. 100/ the rod seems to show six segments (the only 

 such case seen); but it is nearly certain that this represents the 

 X- and F-chromosomes lying end to end, as a separate F-chro- 

 mosome can not be found elsewhere in the nucleus. This case, 

 as well as others where Y is separate, indicates that the F-chro- 

 mosome also may consist of segments, but not more than two 

 such have been seen in any case. Figs. 100, g and h, show two 

 isolated F-chromosomes of the homogeneous type. 



It seems to me hardly possible that this striking appearance is 

 an accidental artifact, first because of the frequency of the tri- 

 segmental type, and second because of the correspondence of the 

 segments of the two halves in each double rod, which is often 

 rendered more striking by a decided inequality of the segments 

 (well shown in fig. 100/) in each half. All such cases that I have 

 seen show the segments accurately paired. For these reasons I 

 believe the segmented structure to be comparable to the linear 

 arrangement of ' chromatin-granules' so often described in the 

 spireme-threads of th^ ordinary chromosomes, and to be an expres- 

 sion of some kind of internal structure in the X-chromosomes. 

 These facts may be added to the evidence reviewed in my preced- 



