17th
7.2 History of cell knowledge The optical microscope was first invented in 17th century. Shortly thereafter scientists began to examine living and dead biological tissues in order to better understand the science of life. Some of the most relevant discovery milestones of the time period include: The invention of the microscope11 , which allowed scientists for the first time to see biological cells Robert Hooke12 in 1665 looked at cork under a microscope and described what he called cork "cells" Anton van Leeuwenhoek13 called the single-celled organisms that he saw under the microscope "animalcules" Matthias Jakob Schleiden14 , a botanist, in 1838 determined that all plants consist of cells Theodor Schwann15 , a zoologist, in 1839 determined that all animals consist of cells Rudolf Virchow16 proposed the theory that all cells arise from previously existing cells In 1838, the botanist Matthias Jakob Schleiden and the physiologist Theodor Schwann discovered that both plant cells and animal cells had nuclei. Based on their observations, the two scientists conceived of the hypothesis that all living things were composed of cells. In 1839, Schwann published 'Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals', which contained the first statement of their joint cell theory.