IN a previous communication the induction of tumours in BALB/c mouse embryo homografts has been described (Peacock, 1962). The tumours arose in a variety of tissues exposed to carcinogens of the polycyclic aromatic hydrocarbon series. The most frequently recorded tumour was a well differentiated squamous carcinoma. A few sarcomas were seen, alone, or accompanying a carcinoma. Adenomas were also present but were not considered as proof of carcinogenic action due to their occurrence in control implants as well. The high incidence of squamous tumours in embryo lung implants was surprising as the commonly occurring tumour of lung in adult mice is an adenoma. In an extensive review of the literature Shimkin (1955) records only eight spontaneously occurring epidermoid tumours of lung in adult mice. The histogenesis of these squamous tumours of embryo lung has now been studied through the re-examination of the original material and by further experiments. Hydrocarbon induced tumour
