Mora C, Tittensor DP, Adl S, Simpson AG, Worm B: How many species are there on earth and in the ocean? PLoS Biol 2011; 9:e1001127. doi: 10.1371/journal.pbio.1001127.
Wilson EB: The Cell in Development and Heredity, 2nd Edition. The MacMillan Co. 1900.http://www.esp.org/books/wilson/cell/2nd/facsimile/