In the late 1600s, Robert Hooke (1635–1703) was the first to see a cell, initially in a section of cork, and then in bones and plants. In 1824, Henri Dutrochet (1776–1847) proposed that animals and plants had similar cell structures. Robert Brown (1773–1858) discovered the cell nucleus in 1831, and Matthias Schleiden (1804–1881) named the nucleolus (the structure within the nucleus now known to be involved in the production of ribosomes) around that time. Schleiden and Theodor Schwann (1810–1882) described a general cell theory in 1839, the former stating that cells were the basic unit of plants and Schwann extending the idea to animals. Robert Remak (1815–1865) was the first to describe cell division in 1855. Chromosomes were named and observed in the nucleus of a cell in 1888 by Wilhelm von Waldeyen-Hartz (1836–1921). Walther Flemming (1843–1905) was the first individual to follow chromosomes through the entire process of cell division.