78 M. M. RHOADES 



elaboration of fiber-producing material. Normally this unique substance is 

 confined to the true centric region, hence chromosomal fibers arise solely 

 from this part of the chromosome. 



It is our belief: (1) that these centric regions produce an over-abundance 

 of fiber-forming material if abnormal 10 is present in the nucleus; (2) that a 

 portion of this substance escapes from the confines of the centric regions and 

 moves distally along the chromosome to produce supernumerary chromo- 

 somal fibers; and (3) that the knobs either stimulate centric activity or else 

 cause the excess fiber-forming substance to move preferentially along knob- 

 bearing arms so that neo-centric activity is first manifested by these arms. 



The failure of the acentric fragment to form chromosomal fibers suggests 

 that the postulated movement of the material from the true centric region 

 occurs after crossing over has taken place. If it happened prior to pachytene, 

 the regions which later constitute the acentric fragments would receive some 

 of this fiber-producing substance which subsequently could form spindle 

 fibers. In support of the above interpretation is the observation that small 

 aggregations of a substance similar in appearance to that located in the true 

 centric region are sometimes found near the distal regions of some chromo- 

 somes at metaphase I and metaphase II. This observation is subject to vari- 

 ous interpretations. But in conjunction with the behavior of acentric frag- 

 ments, it strengthens the hypothesis that the production of neo-centromeres 

 is intimately related to the presence or activity of the primary centric region. 

 It is obvious that the presumed movement of the products of the centromere 

 along the arms of the chromosome has a bearing on the kinetic theory of Posi- 

 tion Effect. 



Evidence has been presented that the abnormal chromosome 10 produces 

 the phenomenon of preferential segregation, and that it also causes the for- 

 mation of neo-centromeres. Are these two phenomena related — does prefer- 

 ential segregation occur as a consequence of neo-centric activity? While no 

 definite answer can be given at this time a tentative hypothesis has been de- 

 veloped. Sturtevant and Beadle (1936), seeking to account for the absence of 

 egg and larvae mortality following single crossovers in paracentric inversions 

 in Drosophila, postulated that the crossover chromatids were selectively 

 eliminated from the egg nucleus. The two spindles of the second meiotic divi- 

 sion in Drosophila eggs are arranged in tandem. Following a crossover within 

 the inverted segment, the tetrad at metaphase I consists of two non-crossover 

 chromatids, a dicentric and an acentric chromatid. 



They assumed that the chromatin bridge arising from the dicentric chro- 

 matid, when the homologous centromeres pass to opposite poles at anaphase 

 I, ties its two centromeres together. The spatial arrangement thus produced 

 is such that the two monocentric chromatids lie nearer the two poles than 

 does the dicentric chromatid. 



The persistence of this relationship into the second division results in a 



