The trophoblast theory of cancer (John Beard, 1857-1924) revisited.

Beard's theory can be restated in a modified form in modern terms in the following way. Cancer represents primarily trophoblastic tissue derived either from an aberrant germ cell or from a somatic cell whose normally repressed 'asexual generation' genes are abnormally reactivated ('derepressed'). The variety of tumors, other than teratomas, may be due to a parallel chance derepression of some genes of somatic ('sexual gneration') characters. This would be a defensive reaction against intramural parasitization by trophoblast and would result in the differentiation and hyperplasia of normally present more primative somatic cells.