86 



p. L. T. ILBERY, P. A. MOOKE, S. M. WINN, AND C. E. FORD 



more than 5% of cells with less than 40 chromosomes or if there are more than 

 3% of cells with greater than 40. When more than one clone is present the 

 clone containing the greater number of cells is listed first. Technically- 

 satisfactory preparations were obtained from the thymuses of 19 animals in 

 the pre-leukaemic phase. Chromosome abnormalities were detected in 12 

 (Table I); of these, 9 thymuses showed an increased chromosome number 



Table I. Irradiated pre-leukaemic mice 



either as the mode or as the presence of clones whilst 4 of these thymic 

 samples had chromosome complements in which a mode of greater than 40 

 chromosomes predominated. In the latter category 1 pre-leukaemic animal 

 whose thymic weight was 64 mg also showed a clone of 41 in the bone- 

 marrow evidencing very early spread. Another mouse in which a mode of 

 39 was observed was discounted as no cells mth the normal complement of 40 

 were found even in bone-marrow and being a female, it was presumed to be a 

 spontaneous XO animal. 



Table II, which contains records of those mice used in the pre-leukaemic 

 experiment found to have at sacrifice a thymic weight of greater than 70 mg, 

 illustrates the changes typical of those we find in radiation-induced leukaemia 

 and supports the conception of the thymus as the originator of the abnormal 

 cell spread to other organs. 



