<P> The nucleus was also described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London . Brown was studying orchids under microscope when he observed an opaque area, which he called the "areola" or "nucleus", in the cells of the flower's outer layer . </P> <P> He did not suggest a potential function . In 1838, Matthias Schleiden proposed that the nucleus plays a role in generating cells, thus he introduced the name "cytoblast" (cell builder). He believed that he had observed new cells assembling around "cytoblasts". Franz Meyen was a strong opponent of this view, having already described cells multiplying by division and believing that many cells would have no nuclei . The idea that cells can be generated de novo, by the "cytoblast" or otherwise, contradicted work by Robert Remak (1852) and Rudolf Virchow (1855) who decisively propagated the new paradigm that cells are generated solely by cells ("Omnis cellula e cellula"). The function of the nucleus remained unclear . </P> <P> 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 . </P> <P> The nucleus is the largest cellular organelle in animal cells . In mammalian cells, the average diameter of the nucleus is approximately 6 micrometres (μm), which occupies about 10% of the total cell volume . The viscous liquid within it is called nucleoplasm (or karyolymph), and is similar in composition to the cytosol found outside the nucleus . It appears as a dense, roughly spherical or irregular organelle . The composition by dry weight of the nucleus is approximately: DNA 9%, RNA 1%, Histone Protein 11%, Residual Protein 14%, Acidic Proteins 65% . </P>

Where is the nucleus located in a plant and animal cell