Most cells are very tiny, beyond the limit of human vision. A microscope must be used to observe the cells. However, until the objective existence of cells is recognized, it is still impossible to know that the objects observed under the microscope are cells. So in 1677, when A. van Leeuwenhoek observed an animal "sperm" with a simple microscope he made, he didn't know that it was a cell. The word cell (derived from the Latin cella, which originally means void, cell) was named after R. Hooker saw that the cork contained small cells when observing the cork section in 1667. In fact, these cells are not living structures, but voids formed by cell walls, but the term cell has been used for this reason. In the enlightenment period of cytology, although many small objects—such as bacteria and ciliates—are also observed with a simple microscope, the main purpose is to observe some developmental phenomena, such as the metamorphosis of butterflies, the structure of sperm and eggs. Due to the limitations of the microscope at the time, the inaccuracy of observations, and the constraints of religious beliefs, these observations actually supported the dogma of pre-establishment theory. Some people claim that they have seen specific and subtle "little men" in sperm, and believe that they will develop into future individuals ---essentialists; others believe that "little men" exist in eggs ---eggists. The influence of preconceptions lasted for more than 100 years, hindering people from further understanding of cells on the basis of R. Hooker. It was not until 1827 that К.M. Bell discovered mammalian eggs that he began to carefully observe the cells themselves. The achromatic objective lens developed before and after this, the introduction of carmine and hematoxylin as dyes to color the nucleus, and the start-up of microtome and slicing technology have created favorable conditions for more detailed observation of cells.
