426 



BREAKDOWN OF GENETIC SYSTEMS 



since the gene changes in question may have depended on structural 

 changes. Nor can we yet say whether the chromosomes divide in 

 different parts at different times as they do at the end of pachytene. 

 New hght is thrown on the time and control of division in the 

 chromosome by the behaviour of an acentric fragment produced 



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Fig. I 28b. — Diagram showing how fragmentation and asym- 

 metrical interchange will occur between chromosomes (left) 

 and between chromatids (right), giving a pseudo-chiasma 

 according to the time of treatment. (After Mather, 1933.) 



at meiosis in Podophyllum (D., 1936). Such fragments produced 

 by crossing-over in inversions are usually lost in the cytoplasm, 

 but this very long fragment, having been included in the nucleus, 

 is seen lying on the spindle at anaphase of the next mitosis. It is 

 remarkable in that, unlike the normal chromosomes, it has failed 

 to divide during the resting stage. It appears, when the pollen 

 grain is rolled over, as a uniform cylinder of the same thickness as 



