种质（英语：Germ plasm）或称为极质，英语：polar plasm）是一些模式生物（例如秀丽隐杆线虫、黑腹果蝇、非洲爪蟾）卵细胞的细胞质中存在的一块区域，其中含有发生生殖细胞世系的决定子。当合子经历有丝分裂时，种质最终仅限于形成胚胎的少数细胞。这些生殖细胞接下来迁移到性腺之中。

Germ plasm or polar plasm is a zone found in the cytoplasm of the egg cells of some organisms, 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.