562 CLARENCE E. McCLUNG 



the only one assumed, and some of the figures would be difficult 

 of conception in the absence of acquaintance with the possible 

 range of chromosome movements. For a number of reasons 

 these variable forms are of importance and require consideration. 

 It is to be noted that this extended ring may be the final shape 

 assumed by other metaphase forms just previous to the ana- 

 phase, or it may represent the status attained by chromosomal 

 movements antecedent to the metaphase. Probably all that is 

 involved throughout is a differerj<?e in relative movements of the 

 chromatids. An examination of figures 146, 16c, plate 3 will 

 reveal a chromosome, one part of which is extended over the 

 spindle as are the members of the flattened ring already de- 

 scribed, while, embraced between its ends, lies another ring 

 within the plane of the equatorial plate. By comparing this 

 chromosome with others shown in photomicrographs Q-V, 

 plate 8 it is seen that this second ring may be progressively 

 reduced until it corresponds in form and position to the extended 

 element to whose ends it is attached — the sole remnant of the 

 equatorial ring being an enlargement at the middle of the curved 

 rod. It is apparent that the enclosed spaces in these two rings, 

 lying at right angles to each other, can not be the same morpho- 

 logically, unless the chromatids in synapsis have shifted. One 

 must represent an interchromosomal space, the other the longi- 

 tudinal spht of the chromosomes. In this respect the element 

 is similar to the double ring figures in certain Orthopteran tetrads. 

 Another form of not infrequent occurrence is. similar in out- 

 line to the hexad of Mermiria and consists of a curved rod applied 

 conformably to the spindle, mth the fibers attached at the ends. 

 From these ends also there spring at various angles more or less 

 straight chromatin rods equal in length, approximately, to half 

 the curved rod to which they join. Superficially such an element 

 is almost the counterpart of the Mermiria hexad, but is nothing 

 more than the large extended ring whose shorter half has divided 

 at the equatorial constriction in advance of the longer (fig. 24). 

 A more puzzhng case is that of the curved rod extending from 

 one pole of the spindle to the other and almost equalling the 

 combined length of the two halves of the ring or V (fig. 19, pi. 



