Between 1877 and 1878 , Oscar Hertwig published several studies on the fertilization of sea urchin eggs , showing that the nucleus of the sperm enter the oocyte and fuses with its nucleus . This was the first time it was suggested that an individual develop from a ( single ) nucleated cell . This was in contradiction to Ernst Haeckel 's theory that the complete phylogeny of a species would be repeated during embryonic development , including generation of the first nucleated cell from a " monerula " , a structureless mass of primordial mucus ( " Urschleim " ) . Therefore , the necessity of the sperm nucleus for fertilization was discussed for quite some time . However , Hertwig confirmed his observation in other animal groups , including amphibians and molluscs . Eduard Strasburger produced the same results for plants in 1884 . This paved the way to assign the nucleus an important role in heredity . In 1873 , August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity . The function of the nucleus as carrier of genetic information became clear only later , after mitosis was discovered and the Mendelian rules was rediscovered at the beginning of the 20th century ; the chromosome theory of heredity was therefore developed .