[Possible role of the cytoplasm in the course of morphogenesis, namely, in the case of twinning].

A number of facts that cannot be interpreted in terms of nuclear genes would seem to be interpretable in terms of cytoplasmic heredity. The hereditary role of the mitochondria has already been demonstrated in molds. The role of the cytoplasm (matrilineal heredity) has also been shown in some phanerogams, and analogous facts have been noted in insects and molluscs. In amphibians, the influence of an alteration of the egg cortical cytoplasm has been shown to reappear in the following generations. This cortical cytoplasm includes the morphological plan of the organism with its bilateral symmetry. In Tatusia novemcincta the twins may be morphologically or chemically different, which may only be explained by an unequal subdivision of a heterogenic cytoplasm. Similar facts are observed in human twinning. Monozygotic twins are usually discordant with respect to congenital malformations (especially symmelia and anencephaly), which may only be interpreted in terms of unequal distribution of cytoplasmic properties.