474 



The Origin of Polyploids 



affected, but in cells in metaphase marked alterations are ob- 

 served in the behavior of the chromosomes. Perhaps these 

 alterations have been shown most clearly in O'Mara's work in 

 Allium Cepa. When onion root tips are treated with colchicine 

 in the proper concentrations, metaphase figures appear to lack 

 the spindle mechanism normally found in dividing cells. This 

 disturbance to the spindle has also been observed by Eigsti and 



Fig. 137. The immediate effects of some chemicals on chromosomes. 

 («) and (6) Cells from onion root tips, (a) Typical effect of colchicine on 

 chromosomes; isolated chromosomes show lack of coiling and differences 

 in sizes. (6) Cell in which the contraction has shortened the chromosomes 

 far below their normal lengths, (c) Cells of maize treated with para- 

 dichlorobenzene which produces an effect very similar to that of colchi- 

 cine. (Courtesy of Dr. J. G. O'Mara; a and b in the Journal of Heredity; 

 c in Stain Technology.) 



others in different genera. With the disappearance of the spindle 

 naturally further division is inhibited, and the cell remains in 

 this metaphase condition for a considerable time. In the mean- 

 time, cells in anaphase or telophase at the time of treatment 

 continue to divide until they pass into the resting stage, whereas 

 cells that had just begun to divide continue to do so until they 

 reach this irregular metaphase condition. In this way metaphase 

 figures gradually accumulate. 



Cells in metaphase undergo some changes that give them an 

 abnormal appearance. The chromosomes contract and thicken 

 gradually and lose entirely their relational coils. The two chro- 

 matids lie side by side but are quite independent of one another 

 except at the centromere, where they remain attached (Fig. 137). 

 Sometimes they separate more or less and sometimes extend out 



