Genetic species. These are groups of inter-breeding population which are productively isolated from one another.

Geo-Botany. Branch of botany dealing with all aspects of relations between plants and the earth’s surface; comprises plant ecology and plant-geography.

Gephyrea. A class of large, marine Annelid worms that possess little or no segmentation and few or no chactae.

Germ-cells. Gametes

Germinal Epithelium. Epithelium lining seminiferous tubules of vertebrates testis, from which during sexual maturity arise the spermatogonia and the Sertoli cells that presumably nourish the developing sperm, applied also the epitheliums covering coelomic surface of vertebrate ovary, which probably forms follicle cells, though it is now doubtful whether it forms oogonia throughout sexual maturity.

Germination. The beginning of the growth of a seed, spore or other structure that is dormant.

Germ-Layer. (Zool) One of the main layers or groups of cells which can be distinguished in an embryo during and immediately after gastrulation. These layers are roughly similar in arrangement, and in ultimate differentiation, in most animals. There are two in diploblastic animals, endoderm and ectoderm. In triploblastic animals there is in addition the mesoderm.

Germ-plasm. A particular sort of protoplasm which Weismann (1834-1914) suggested was transmitted substantially unchanged from generation to generation via the germ-cells, giving rise in each individual to the body cells (soma) but itself remaining distinct and unaffected by the environment of the individual (theory of continuity of the germplasm).

A special cytoplasm in the ovum which during embryonic development becomes restricted to future germ-cells of adult, a sort of a germ-plasm which does not however give rise to the soma, has been demonstrated in some animals, though not in others.