Induction of Melanotic Lesions of the Iris in Rats by Urethane given during the Neonatal Period

THERE have been a number of reports in recent months of experiments in which animals have been exposed to chemical carcinogenic agents during the neonatal period 1–6 . It is probable that, on the basis of dose per unit of body-weight, newly born animals are particularly sensitive to tumour-induction. For example, lung tumours may be very readily induced in mice by the neonatal injection of small doses of certain weakly carcinogenic agents 4 . Because of this high sensitivity, the use of newly born animals for screening for carcinogenicity has been suggested 3 . Of possibly even greater interest, however, is the fact that animals injected when newly born with carcinogenic agents develop varieties of neoplasms not seen when the same agents are injected into adults of the same species and strain. For example, newly born ‘101’ strain mice injected with small doses of 9,10-dimethyl-1,2-benz-anthracene developed kidney tumours 3 . A further example of this type of phenomenon has recently come to light in an experiment in which rats were injected during the neonatal period with urethane (ethyl carbamate).