How did developments in cytology in the 19th century contribute to our modern understanding of genetics?

Developments in cytology in the 1800s led to the identification of parts of the cell (e.g., cell nucleus and chromosomes). The cell theory caught the attention of biologists during that time and the focus shifted to the study of the cell, which eventually led to the conclusion that the nucleus contains the hereditary information.

Source: Pierce, B. (2012). Genetics: A conceptual approach (4th ed.).