94 p. C. ROLLER, E. LEUCHAES, C. TALUKDAR, AND V. WALLIS 



number of dividing cells as a percentage of the total number of cells counted. 

 The sample used for determining the mitotic index (M.I.) was obtained from 

 colcemid treated animals. 



CHROMOSOMES IN PRIMARY LEUKAEMIAS OF AK MICE 



The high leukaemic strain used for this study was the AK strain, main- 

 tained by Dr. Miller in our Institute. About 85 to 90% of these mice develop 

 thymic lymphoid leukaemia at 9 months of age. The chromosome constitu- 

 tion was analysed in seven AK mice. Cells for this study were obtained from 

 thirty sites, yielding a total number of 836 cells, out of which 72 cells (8-6%) 

 contained more than 40 chromosomes, i.e. deviated from the normal diploid 

 complement. The details are given in Table I. 



The chromosomes of normal mouse cells and host leukaemic cells are 

 acrocentric. Thus the extra chromosomes in cells with 41 or 42 chromosomes 

 cannot be identified. The only alteration in chromosome structure which was 

 observed, was in mouse 303, in which a metacentric marker chromosome was 

 present in all the sites analysed. 



A comparison of the mitotic indices of ceU populations in various sites 

 suggests that the mitotic rate was highest in the thymus, followed by the 

 spleen and marrow. It is interesting to note that the frequency of cells with 

 40-1-1 chromosomes was often highest in tissues with the highest mitotic 

 index. Thus in mouse No. 303, the M.I. of spleen was 4-7, and 11 cells out of 

 40 had more than 40 chromosomes; while in mouse No. 301, the M.I. of the 

 spleen was 2-0 and only one cell out of 43 had an abnormal chromosome 

 number. 



Another interesting finding is the lack of correlation between the age of the 

 animals and the progression of leukaemia as indicated by the peripheral 

 blood count. The 12-month-old mouse (No. 302) had a white blood cell 

 count (W.B.C.) of 25,000 while a six-month-old mouse (No. 250) had a W.B.C. 

 of 41,000. Similarly no relationship is apparent between the M.I. in the 

 marrow and the number of white cells in the peripheral blood; No. 300 had a 

 W.B.C. of 25,800, the M.I. being 0-5 in the marrow, while No. 101 had 19,400 

 white cells and an M.I. 2-0. 



CHROMOSOMES IN VIRUS-ACCELERATED LEUKAEMIA IN AK 



MICE 



Cell-free extract was made from AK leukaemic tissues by Dr. Miller and 

 injected into newborn mice. It has been demonstrated by Rudali et al. (1956) 

 that by such treatment the long latent period of leukaemia could be consider- 

 ably reduced. Up to the time of this report three such accelerated leukaemias 



