[Current concepts about the determinants of germ cells]

The cytoplasm of the egg vegetal hemisphere plays the determining role in the isolation and differentiation of germ cells. Cytoplasmic factors of unknown nature prevent certanin blastomeres from somatic differentiation. In the developing germ cells, a large amount of material (in which acid proteins are a constant component) passes from the nucleus to the cytoplasm. This fine fibrous material may take the form of nucleolus-like bodies or dense granules (polar granules in the insect eggs, dense bodies in the amphibian germ plasm, intermitochondrial cement in oocytes of various animals, chromatoid body in spermatocytes). The application of the term "determinants of germ cells" for designation of these structure, as it takes in the electron microscopy literature, appears to be incorrect. Their appearance is a consequence of the determining influence of cytoplasmic factors, rather than their cause. In lower invertebrates, the germ cells may arise in adult specimens due to the embryonic reserve of totipotent elements (neoblasts or interstitial cells). The transformation of these cells in the germ ones is accompanied by the release of a great amount of fine fibrous material from the nucleus to the cytoplasm as well. The problem of determinants of germ cells is considered with respect to the problem of ooplasmic segregation.