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Essays in Biochemistry 



foster mothers he had available, including a number of older mice. 

 The second experiment also failed to throw any light on the trans- 

 missibility of the disease, but MacDowell did observe that the inci- 

 dence of leukemia was somewhat diminished in those C58 mice which 

 had been nursed on old Storrs-Little mothers. This observation was 

 verified in a third experiment wherein old foster-mother Storrs-Littles 



Fig. 2. Electron micrograph of a preparation from a 4-day-old culture of a spon- 

 taneous mammary tumor explant fixed over osmic acid shadowed with gold at 

 tan- 1 10°. Magnification 17.500X (Porter and Thompson 14 ). 



were used exclusively. 15 These experiments established that the milk 

 of old Storrs-Little mice contains a substance which is either prevent- 

 ative or curative of leukemia. Leukemia, like mammary carcinoma, 

 is a disease of the adult mouse. The transmission of the causative 

 agent, whatever it might turn out to be, occurs antenatally or in 

 nursing, probably the former. Leukemogenesis takes place over a 

 period of time. The milk inhibitor was given early in this period, 

 that is, before the leukemic state. It can be inferred then that Mac- 

 D< >well's substance is prophylactic rather than therapeutic. Experi- 

 ments on the isolation and characterization of the substance are still 

 in progress at this writing, but it seems reasonable to suppose that 

 this substance acts by compensating for a deficiency produced by the 

 leukemogenic mutagen. 



