SINGLE AND DOUBLE RINGS. 



lacing at the junction, so that a ring or V gradually diminishes 

 in size as its chromatids are pulled into the loop by the spindle 

 fibers. 



3. Whether, in vertical rings or Y's, the process is the same as 

 the above; or whether the upper and lower halves of the rings 

 or V's separate as wholes. 



4. What different configurations are shown by the same homo- 

 logous chromosomes in different cells. 



BIVALENT I. 



Chromosome I., in the pollen grain (Fig. i), is seen to be formed 

 of two slightly unequal segments, and is usually bent; the spindle 



FIG. i. This shows the metaphase at the first division in the pollen-grain. 

 (Like all the other figures it was made from preparations of Uvularia grandiflora.) 

 This pollen-grain was selected from a large number as showing the chromosomes 

 well spaced. The longitudinal divisions and the constrictions are evident. It can 

 be seen that the two segments of chromosome I. are of unequal length. Chromo- 

 some III. has apparently a shorter long segment and a longer short segment than 

 chromosome II. The small segment of chromosome V. is closely attached to the 

 large segment, while in chromosome VI., the two segments are separated by a 

 thread. 



This, and the subsequent figures were drawn with the camera from iron-aceto- 

 carmine preparations, the light being screened by Wratten yellow-green filter No. 

 56. A Leitz' achromatic aplanatic condenser, corrected for use as a water-im- 

 mersion, was used to give a large cone of light. Zeiss' apochromatic water- 

 immersion 70 was employed in all cases. 



fiber being, of course, attached at the constriction, where the 

 two segments are connected by a fine thread. Fig. 2 of the paper 

 on the origin of chromosomal mutations (Belling, 19256) shows 

 bivalent I. in the late prophase in the form of two medium-sized 



