The germ plasm (or polar plasm) is a zone found in the the cytoplasm of the egg cells of some model organisms (such as C. elegans, Drosophila, Xenopus), which contains determinants that will give rise to the germ cell lineage. As the zygote undergoes mitotic divisions the germ plasm is ultimately restricted to a few cells of the embryo, these germ cells then migrate to the gonads.

The german biologist August Weismann formulated the germ plasm theory in the 1890's, in which he stated that the germ plasm was the essential nuclear part of germ cells, that it remained qualitatively unchanged from the zygote (in contrast with somatic cells) and was responsible for heredity.

See also: developmental biology.