Germ Track

the series of generations of cells from the primordial germ cells of the embryo to the sexual products of the adult organism.

The concept of the germ track was most fully elaborated by A. Weismann in 1892 in order to ground his theory of the continuity of the germ plasm. Weismann believed that because of the genetically uneven division of the nucleus in the earliest stages of the development of an individual there is differentiation of the germ cells, which, in contrast to the cells of the body (the somatic cells), contain all the genetic potentials of the ovum; the germ cells form a continuous line of germ plasm, the potentially “immortal” part of the body. The notion of the impoverishment of the genotype of the somatic cells because of genetically unequal division turned out to be unfounded.

Experiments in transplanting nuclei have shown that in amphibians, for example, all nuclei have equal genetic potential until the gastrula stage. Relatively early emergence of germ cells has been traced in many animals and in man. Thus, in ascarids, crustaceans, insects, and frogs the primor-dial germ cells are isolated as early as the process of cleavage. In reptiles and birds they arise in the gastrula stage; in mammals and man, in the process of neurulation and early organogenesis. When the primordial germ cells are destroyed by radiation or cautery (for example, in anthropods, amphibians, or birds), sterile gonads are formed. In animals with early isolation of germ cells, the primordial germ cells are the sole source of sexual products. In all vertebrates the coelomic epithelium of the gonads (the so-called germinal epithelium) apparently does not form germ cells. Primordial germ cells, which arise outside the gonads (long before gonadal formation), settle in the coelomic epithelium after a complex migration. The primordial germ cells have a number of distinct morphological and biochemical characteristics by comparison with the somatic cells: in mammals they are distinguished by high alkaline-phosphatase activity; in some reptiles, by a large number of polysaccharides, and so forth. In insects, lower crustaceans, and certain amphibians, in a special section of the cytoplasm of the dividing ovum, there are inclusions that are rich in ribonucleic acid (polar plasma, germ cytoplasm, octosomes, sex determinants) and are later discovered only in the cytoplasm of the primordial germ cells. It is they, apparently, that determine the subsequent development of the primordial germ cells.

The early isolation of germ cells is considered a particular case of early cell specialization (along with the early differentiation of somatic foundations, such as the chordamesoderm in amphibians).