TIME OF SPLIT 425 



out by direct observation whether this is so, since the finer details 

 of structure are concealed in the resting nucleus. We can, however, 

 resolve the question indirectly by X-ray treatment (Mather and 

 Stone, 1933 ; cf. Lewitsky and Araratian, 1932, and Huskins and 

 Hunter, 1935). 



Thus when chromosomes are broken or interchanged under 

 X-ray treatment before they have divided, all the changes produced 

 will be exactly paired in the chromatids seen afterwards at meta- 

 phase. These changes in whole chromosomes may be described as 

 due to chromosome-breaks. When, on the other hand, the cells 

 are treated after their chromosomes have divided the changes they 

 undergo will be changes in their chromatids ; each chromatid will 

 be broken or interchanged independently of its partner. These 

 two conditions should be distinguishable at the following metaphase. 

 Now in the higher plants, the resting stage between meiosis and the 

 following mitosis in the pollen-grain continues for several days or 

 weeks. In Tradescantia the interval is about one week. When, 

 therefore, metaphases of mitosis are examined in the pollen-grain, 

 three, two, or one day after X-raying, it is known that the nuclei 

 giving rise to these metaphases have been treated at successively 

 later stages of development of the resting nucleus. 



Riley {1936) and Faberge and Mather (unpublished) have con- 

 firmed these assumptions and shown that before a certain period 

 all the changes are from chromosome breaks, and after this period 

 they are all from chromatid breaks. The critical period is different 

 in different species, twenty-four to thirty-six hours before mitosis 

 in Tradescantia, earlier in Allium, always, however, some time before 

 the end of the resting stage, which it cannot, therefore, be supposed 

 to determine (Fig. 128 b). An exactly corresponding conclusion can 

 be drawn from the mutations induced in ripe sperm in Drosophila. 

 One-sixth of these produce mosaic offspring, one side mutant, the 

 other non-mutant, owing to the genes concerned having divided 

 before they were hit and the altered and unaltered daughter-genes 

 having passed to opposite cells in the first cleavage division of the 

 fertilised egg (Patterson, 1933). It might seem, therefore, that 

 the genes, and the thread which connects them, divide at the same 

 time. This does not follow without further evidence, however. 



