EXPERIMENTAL LEUKAEMIA 



leukaemia which are relevant in the present connection as 



follows : 



(i) Different pure strains of mice differ greatly in their 

 spontaneous incidence of leukaemia and in the ease 

 with which leukaemogenic agents can induce the 

 condition, 

 (ii) With suitable mouse strains, a wide variety of agents, 

 notably ionizing radiation, carcinogens of methyl- 

 cholanthrene type, and oestrogenic hormones, can be 

 shown to increase the incidence of leukaemia, almost 

 always of lymphocytic type. 

 (iii) If appropriate host and donor strains of mice are used, 

 it is possible to induce the subsequent appearance of 

 leukaemia by inoculation of new-born mice with non- 

 cellular extracts from certain types of leukaemia and 

 malignant tumour (Gross, 1951). 

 (iv) A similar agent, active in transmitting leukaemia to 



older mice, has been described by Friend (1957). 

 (v) In many strains the presence of thymus is necessary 

 for the induction of leukaemia by X-rays or carcino- 

 gens. An indirect mechanism of irradiation-leukaemo- 

 genesis is postulated by Kaplan (1954), since it can be 

 shown that, in some experiments, tumours appearing 

 in thymectomized irradiated mice grafted (after irradia- 

 tion) with normal thymus arose in the thymus and 

 were genetically of the donor type. Law confirms this, 

 but only for a minority. In most experiments the 

 tumours appeared in the graft area but were genetically 

 of the host type. 

 (vi) Virus-like objects of rather characteristically variable 

 size have been found in many mouse tumours 

 and leukaemic cells by electron microscopy (see 

 Dmochowski, 1957). 

 One's first comment would be that there is an air of 



biological unreality about much mouse leukaemia work, 



simply because all transmission experiments are by parenteral 



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