362 JOHN BELLING. 



without splitting lengthways. The constriction between the 

 short and long segments is visible. The upper line of Fig. 5 shows 

 a different configuration for bivalent IV., in which the two sides 

 of the loop are equal. This is intelligible, first if we suppose the 

 point of attachment of the spindle fiber and the constriction to 

 have been shifted to the center of both chromosomes, which 

 seems unlikely. Or we may presume that a horizontal V has 

 separated into its chromatids, somewhat as bivalent III. of Fig. 6. 



BlVALEXTS V. AND VI. 



Chromosomes V. and VI., since they differ only in VI. having 

 its small segment attached by a long filament, cannot be told 

 apart in the maturation divisions, and must therefore, be con- 

 sidered together. In Fig. 2 of the paper on the origin of muta- 

 tions, bivalents V. and VI. have the form of V's or truncated A's. 

 In Fig. 3 of the present paper, in the configuration marked (V.), 

 the two small segments have separated, and the two large seg- 

 ments are still horizontal and show the split between the chro- 

 matids. Similar forms may be seen in the top line of Fig. 5. 

 The bivalent marked (VI.) in Fig. 3, however, like the one in 

 the bottom line of Fig. 4, is presumed to have come from the 

 separation of the chromatids of a horizontal V. 



BIVALENT VII. 



Finally bivalent VII. resembles* V. and VI. in its configurations, 

 but is recognizably smaller. 



To sum up: (i) There is, apparently, fusion at the junctions 

 of the two constituents of each bivalent. (2) Horizontal rings 

 and V's divide into their constituent chromatids along a hori- 

 zontal plane, the line of division being distinct at the metaphase. 

 (3) Vertical V's and rings have not been observed in the act of 

 separating; but they probably separate into upper and lower 

 halves. (4) The rings and V's apparently diminish as the loop 

 between them is pulled out by the spindle fibers. This would 

 presuppose crossing or interlacing of the chromatids. 



