Weismann advocated the germ plasm theory , according to which ( in a multicellular organism ) inheritance only takes place by means of the germ cellsâ '' the gametes such as egg cells and sperm cells . His main contribution was the germ plasm theory , according to which ( in a multicellular organism ) inheritance only takes place by means of the germ cellsâ '' the gametes such as egg cells and sperm cells .