A worksheet function that is called by Excel can call back into Excel using the C API. An XLL command that is called by Excel can call the C API. DLL and XLL functions and commands that are called by VBA when VBA has itself been called by Excel can call the C API.
Cell Theory. The discovery of the cell was made possible by the invention of the microscope, which was made possible by improved lens-grinding techniques.Antoni van Leeuwenhoek (1632-1723), a Dutch tradesman, learned to grind lenses and assemble them into simple microscopes.nspired by Hooke's work, Leeuwenhoek began making microscopic examinations of his own. In 1678, he reported to the Royal Society that he had discovered little animals -- bacteria and protozoa -- in various samples. The society asked Hooke to confirm Leeuwenhoek's findings, and he did.
In 1838 Schwann became familiar with Matthias Schleiden's microscopic research on plants. Schleiden described plant cells and proposed a cell theory which he was certain was the key to plant anatomy and growth. Pursuing this line of research on animal tissues, Schwann not only verified the existence of cells, but he traced the development of many adult tissues from early embryo stages.