The lung tissue of mouse embryos of the C strain proliferates to some extent after implantation in adult hosts of the same breed and rapidly differentiates, forming a parenchyma remarkably like the normal. The grafts persist long. When methylcholanthrene dissolved in olive oil has been introduced with them much more growth of them occurs. The carcinogen induces a pronounced metaplasia of the epithelium of the bronchial tree, and the altered cells are often aggressive, multiplying, invading, and largely replacing the parenchyma about them. So closely do they resemble malignant elements in aspect and behavior that it is frequently difficult to tell whether carcinomatous change is not actually present. Genuine tumors soon arise, multiple benign adenomas sometimes appearing within 3 weeks, and indubitable carcinomas a few weeks later. Not a few of the cancers are of transitional cell type, that is to say are expressive of an intermediate stage in the metaplasia.
Under the influence of methylcholanthrene the cells lining the alveolar spaces of the graft sometimes undergo metaplasia also, with result in stratified squamous epithelium. It follows that there is reason to doubt the current assumption that all squamous cell carcinomas of the lung necessarily arise from the bronchial tree. The findings, taken with others previously reported, make it difficult to suppose, furthermore, that the generality of lung tumors can be due to neoplastic viruses entering the organism in postnatal life and having no broader scope than those thus far discovered.