In the late nineteenth and early twentieth centuries, germ cells acquired a central position in the evolutionary debate. This holds true principally for the underlying conceptual implications of the germ plasm theory rather than for germ cells proper, which, on the contrary, soon faded away from the mainstream research focus. The main theoretical value consisted in opening a new approach to the investigation of the mechanisms of inheritance and development, in linking this approach with the issues of the contemporary evolutionary debate, and in becoming a decisive argument in the battle between neo-Lamarckians and neo-Darwinians. The former defended the possibility of inheritance of acquired characters (soft inheritance), the latter maintained that natural selection was sufficient to explain evolutionary change. Weismann was able to establish a direct lineage of germ cells, which went through all stages from the fertilized egg cell to the adult sex cells and remained always distinct from the somatic cells.