Table 1.

Glossary of terms

Germline	Translated from the German ‘Keimbahn’, also translated sometimes as ‘germ track’; it is the line of cells that carry the genetic material from one generation to the next
Germ plasm	A collection of determinants often found in the oocyte of several animals and which are often inherited by the germline
Nuage	A wider term (literally ‘cloud’ in French) referring to the electron-dense perinuclear material or granules often found in multipotent and germline cells and whose composition is similar to the germ plasm. This material is known by many different names in different organisms: polar granules in Drosophila melanogaster, P granules in Caenorhabditis elegans, chromatoid body and intermitochondrial cement in mammals, chromatoid bodies in planarians, Balbiani bodies in many vertebrates and invertebrates, or germ granules in a more generalized way
Germline Multipotency Program (GMP)	A term recently coined by Juliano and co-workers and referring to a collection of genes often expressed in nuage-containing multipotent cells and germline cells. Composed of genes known to be involved in germline determination, germ plasm and nuage assembly, these genes are often also found in multipotent or pluripotent cells. Examples of these are vasa, nanos, piwi, tudor, pumilio and bruno genes, for instance
Primordial Germ Cells (PGCs)	The first cells in the developing germline to have only germ potential. They undergo mitotic expansion and later in development populate the gonads to give rise to germ cells
Germ Cells (GCs)	In a sexually-reproducing animal, the cells that give rise to the gametes in the gonads
Preformation	The process of germ cell specification that proceeds via zygotic maternally-deposited germ plasm components which are selectively inherited by germline cells. The specification signal is therefore intrinsic
Epigenesis	The process of germ cell specification that proceeds via signals that segregate a population of cells from a multipotent or pluripotent group of cells. The specification signal is, therefore, extrinsic
Weismann barrier	Derived from August Weismann’s theory that genetic information only flows from germline cells to somatic cells, and it cannot flow from somatic cells to the germline. In other words, changes in the germline genetic information can affect somatic tissues and can be passed on to the next generation, while somatic mutation can affect neither the germline nor the forthcoming generations