The instrument that was crucial in the development of cell theory during the 18th and 19th centuries was the microscope.

Here's why:

* Magnification: Microscopes allowed scientists to see objects too small to be visible to the naked eye, like cells.

* Observation: Through the microscope, scientists like Robert Hooke, Antonie van Leeuwenhoek, and Matthias Schleiden could observe the structure of plant and animal tissues, leading to the discovery of cells.

* Detailed Study: Microscopes enabled scientists to study the internal components of cells, like the nucleus, and understand their functions.

While other instruments were used in scientific research during that time, the microscope was instrumental in the advancement of cell theory.