SINGLE AND DOUBLE KI.V,^. 



upper line of Fig. 4, the ring and V. In Fig. 5, lower line, In >i h 

 short and long segments have been drawn into the loop. Fig. 6 is 

 especially instructive, for bivalent II. probably had consisted of 



j .. 



FIG. 6. A rare stage of the pollen-mother-cell at the early anaphase, showing 

 the final separation of bivalents I., II. and III. The arrangement of the chromatids 

 of bivalent II. must apparently be due to the horizontal splitting of a horizontal 

 ring. The four smaller bivalents are, as usual, ahead of the larger ones in com- 

 pleting the separation of their constituents. 



a large horizontal ring with a vertical piece formed of the two 

 short segments. This is seen to have separated by the horizontal 

 splitting of the ring into its constituent chromatids, the small 

 segments having first separated. In bivalent III. of Fig. 6, a 



horizontal V has, it seems, separated into chromatids in the 

 same way. 



BIVALENT IV. 



Chromosome IV., as shown in Fig. i, consists of a small J, in 

 which the two segments are close together. It is distinctly 

 larger than either V. or VI. In Fig. 2 of the paper on the origin 

 of mutations, bivalent IV. is formed of a horizontal V, with a 

 vertical piece consisting of the two short segments. In Fig. 2 of 

 the present paper, this bivalent, which apparently started as a 

 ring, is already in the early anaphase (for the small bivalents 

 usually separate before the large ones). In Fig. 3, bivalent IV., 

 which possibly started as a ring and V, still shows a small V at i In 

 left of the loop. A later stage is shown in the lower line of Fig. 5. 

 In the top of Fig. 4, the two halves of bivalent IV. have separated 



