Petridish filling units

Petri dishes are named after German physician Julius Petri. In the late 1880s Petri developed a set of nesting glass plates that created an ideal environment for growing microorganisms. The deep, flat dish filled with a nutrient-rich gelatin provided a place for growth. The lid protected the sample from contamination and facilitated its viewing under a microscope. 

From 1877 to 1879 he was assigned to the Imperial Health Office (Kaiserliches Gesundheitsamt) in Berlin, where he became an assistant to Robert Koch. On the suggestion of Angelina Hesse, the New York-born wife of another assistant, Walther Hesse, the Koch laboratory began to culture bacteria on agar plates. Petri then invented the standard culture dish, or Petri plate, and further developed the technique of agar culture to purify or clone bacterial colonies derived from single cells. This advance made it possible to rigorously identify the bacteria responsible for diseases