 Between 1877 and 1878 , Oscar Hertwig published several studies on the fertilization of sea urchin eggs , showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus . This was the first time it was suggested that an individual develops 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 were rediscovered at the beginning of the 20th century ; the chromosome theory of heredity was therefore developed . 
 Between 1877 and 1878 , Oscar Hertwig published several studies on the fertilization of sea urchin eggs , showing that the nucleus of the sperm <a href='uninflect' title='1'><mark>enters</mark></a> the oocyte and fuses with its nucleus . This was the first time it was suggested that an individual <a href='uninflect' title='2'><mark>develops</mark></a> from a ( single ) nucleated cell . This was in contradiction to Ernst Haeckel <a href='null_genetive' title='3'><mark>'s</mark></a>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 <a href='uninflect' title='4'><mark>were</mark></a> rediscovered at the beginning of the 20th century ; the chromosome theory of heredity was therefore developed . 
Between 1877 and 1878 , Oscar Hertwig published several studies on da fertilization of sea urchin eggs , showing dat the nucleus of the sperm enter the oocyte and fuses with its nucleus . dis was the first time it was suggested tdatan individual develop from a ( single ) nucleated cell . Tdiswas in contradiction to Ernst Haeckel  theory thdathe complete phylogeny of a species would be repeated durin embryonic development , including generation of tdafirst nucleated cell from a " monerula " , a structureless mass of primordial mucus ( " Urschleim " ) . Therefore , thdaecessity of thedaerm nucleus fa fertilization was discussed ffaquite sum 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 daleus an important role in heredity . In 1873 , August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity . The fdation 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 .