146 EZRA ALLEN 



the right-hand pair more than the left, but that union is still 

 preserved along a large portion of the structure. This arrange- 

 ment of parts persists while the chromatin becomes steadily- 

 more condensed and the chromosomes assemble to form the 

 metaphase plate. The appearance of chromosome B is shown in 

 figure 37 in partial side view, and in figure 18 viewed from above. 



The history ends with division, by which this tetrad is separated 

 into two simple V's on the spindle, as shown in figure 42. Be- 

 tween this stage and that shown in figure 35, the movements of 

 the chromatids have been as follows : the sides of the ellipse have 

 approximated along their whole length, the spindle fibers have 

 been attached at the ends (on the right in the figures), and the 

 tetrad has stretched out gradually along the spindle, assuming 

 progressively the forms shown in figures 37 to 40. Figure 40 

 shows almost the last stage previous to division. Just following 

 this condition the hump disappears completely and the chromo- 

 some seems to be a single rod. Its composition cannot be dis- 

 cerned, but just as soon as the central ends separate, the Vs 

 appear. At first their sides are very closely approximated, as 

 shown in figure 41, but they quickly spread apart and appear as 

 in figures 42 and 22. 



The history of this chromosome may be regarded as typical of 

 all the ring-shaped forms. The more complex ring and the 

 sunple rings vary in slight details only. The chromatid arrange- 

 ment in the triple ring has not been seen in diakinesis, but 

 as indicated by figures 18, 19, 32, 33, and by figures 40 to 42, 

 (the last three serve as well for this chromosome as for B) , there 

 is no reason why it should be interpreted differently. The sim- 

 ple ring is illustrated in figures 18 (D), and 27 {D and several 

 small ones). 



Figures 27 (C), 35, 37, 39 and 51 are typical of the large ring in 

 which the pairs of chromatids separate at both ends. It would 

 seem that in some of the medium-sized and smaller rings this 

 double separation does not occur, as the lugs are seen at only 

 one end. (Figs. 18 and 27). 



In the case of the cross, the V which results in anaphase is 

 fonned by the separation of two pairs of arms, shown in figure 29 



